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/gtk/install.txt

@@ -145,61 +145,6 @@ make install
 ldconfig
 exit
 
-* Building wxGTK on OS/2
-------------------------
-
-Please send comments and question about the OS/2 installation
-to Stefan Neis <Stefan.Neis@t-online.de> and patches to
-the wxWidgets mailing list.
-
-In the following list, the version numbers indicate the configuration that
-was actually used by myself, newer version should cause no problems and
-even older ones are expected to work most of the time.
-
-You'll need OS/2 Warp (4.51) or eCS(1.0), X-Free86/2 (3.3.6 or newer),
-GTK+ (1.2.5 or newer), emx (0.9d fix 4), a Unix like shell (pdksh-5.2.14 or
-ash), Autoconf (2.57), GNU file utilities (3.13), GNU text utilities (1.19),
-GNU shell utilites (1.12), m4 (1.4), sed (2.05), grep (2.0), Awk (3.0.3),
-GNU Make (3.75).
-
-Preferably, you should have Posix/2 installed and C(PLUS)_INCLUDE_PATH and
-LIBRARY_PATH set up accordingly, however, wxGTK will even work without it.
-Presence of Posix/2 will be auto-detected.
-
-Open an OS/2 prompt and switch to the directory above.
-Set MAKESHELL or MAKE_SHELL (which one is needed depends on the version of
-make) to a Unix like shell, e.g.
-SET MAKESHELL=ash
-If you have a really deficient version of GNU make, it might even be
-necessary to set SHELL or even COMSPEC to a unix like shell as well. 
-Depending on your installation you might want to also set INSTALL, for me
-it tends to try to use the system's tcpip\pcomos\install.exe which causes
-problems, e.g.
-SET INSTALL=<path_to_src_directory>/install-sh -c
-
-Notice that the delivered configure scripts are fully OS/2 aware, so you
-can simply run
-    ash -c "configure --with-gtk=1"
-and make and possibly make install as described above.
-
-* Building wxGTK on SGI
------------------------
-
-Using the SGI native compilers, it is recommended that you
-also set CFLAGS and CXXFLAGS before running configure. These
-should be set to :
-
-CFLAGS="-mips3 -n32"
-CXXFLAGS="-mips3 -n32"
-
-This is essential if you want to use the resultant binaries
-on any other machine than the one it was compiled on. If you
-have a 64bit machine (Octane) you should also do this to ensure
-you don't accidently build the libraries as 64bit (which is
-untested).
-
-The SGI native compiler support has only been tested on Irix 6.5.
-
 * Building wxGTK on Cygwin
 --------------------------
 
@@ -326,11 +271,6 @@ drastically reduced by removing features from wxWidgets that
 are not used in your program. The most relevant such features
 are
 
-    --with-odbc             Enables ODBC code. This is disabled
-                            by default because iODBC is under the
-                            L-GPL license which is less liberal than
-                            wxWindows licence.
-
     --without-libpng        Disables PNG image format code.
 
     --without-libjpeg       Disables JPEG image format code.