Remove obsolete symbols from overviews and install notes

Don't mention Windows 95, OS/2 and SGI.

Closes https://github.com/wxWidgets/wxWidgets/pull/135

docs/doxygen/mainpages/platdetails.h

@@ -164,7 +164,7 @@ the border.
 
 If you don't specify a border style for a wxTextCtrl in rich edit mode,
 wxWidgets now gives the control themed borders automatically, where previously
-they would take the Windows 95-style sunken border. Other native controls such
+they would take the sunken border style. Other native controls such
 as wxTextCtrl in non-rich edit mode, and wxComboBox already paint themed
 borders where appropriate. To use themed borders on other windows, such as
 wxPanel, pass the @c wxBORDER_THEME style, or (apart from wxPanel) pass no

docs/doxygen/overviews/fontencoding.h

@@ -28,9 +28,7 @@ wxFontEnumerator and wxFontMapper. wxFont encoding support is reflected by a
 
 @beginDefList
 @itemdef{wxFONTENCODING_SYSTEM,
-        The default encoding of the underlying
-        operating system (notice that this might be a "foreign" encoding for foreign
-        versions of Windows 9x/NT).}
+        The default encoding of the underlying operating system.}
 @itemdef{wxFONTENCODING_DEFAULT,
         The applications default encoding as returned by wxFont::GetDefaultEncoding.
         On program startup, the applications default encoding is the same as

docs/doxygen/overviews/toolbar.h

@@ -32,23 +32,6 @@ The following is a summary of the toolbar classes and their differences:
 
 @li wxToolBarBase: This is a base class with pure virtual functions, and should
     not be used directly.
-@li wxToolBarSimple: A simple toolbar class written entirely with generic
-    wxWidgets functionality. A simple 3D effect for buttons is possible, but it
-    is not consistent with the Windows look and feel. This toolbar can scroll,
-    and you can have arbitrary numbers of rows and columns.
-@li wxToolBarMSW: This class implements an old-style Windows toolbar, only on
-    Windows. There are small, three-dimensional buttons, which do not
-    (currently) reflect the current Windows colour settings: the buttons are
-    grey. This is the default wxToolBar on 16-bit windows.
-@li wxToolBar95: Uses the native Windows 95 toolbar class. It dynamically
-    adjusts it's background and button colours according to user colour
-    settings. CreateTools must be called after the tools have been added. No
-    absolute positioning is supported but you can specify the number of rows,
-    and add tool separators with @c AddSeparator. Tooltips are supported.
-    @c OnRightClick is not supported. This is the default wxToolBar on Windows
-    95, Windows NT 4 and above. With the style wxTB_FLAT, the flat toolbar look
-    is used, with a border that is highlighted when the cursor moves over the
-    buttons.
 
 A toolbar might appear as a single row of images under the menubar, or it might
 be in a separate frame layout in several rows and columns. The class handles
@@ -71,11 +54,7 @@ give it.
 
 @section overview_toolbar_library Using the Toolbar Library
 
-Include @c "wx/toolbar.h", or if using a class directly, one of:
-
-- @c "wx/msw/tbarmsw.h" for wxToolBarMSW
-- @c "wx/msw/tbar95.h" for wxToolBar95
-- @c "wx/tbarsmpl.h" for wxToolBarSimple
+Include @c "wx/toolbar.h"
 
 An example of using a toolbar is given in the "toolbar" sample.
 

docs/doxygen/overviews/unicode.h

@@ -126,8 +126,7 @@ Since wxWidgets 3.0 Unicode support is always enabled and while building the
 library without it is still possible, it is not recommended any longer and will
 cease to be supported in the near future. This means that internally only
 Unicode strings are used and that, under Microsoft Windows, Unicode system API
-is used which means that wxWidgets programs require the Microsoft Layer for
-Unicode to run on Windows 95/98/ME.
+is used.
 
 However, unlike the Unicode build mode of the previous versions of wxWidgets, this
 support is mostly transparent: you can still continue to work with the @b narrow