Remove wxALWAYS_SHOW_SB-related code from wxScrolled

There is no need to handle this style specially here, it's supposed to
be handled at wxWindow level and is, indeed, at least in all the major
ports.

So revert 2119b213e3 (see #13616) and the
workaround for it applied later for macOS (see #14856). And this also
removes the need for handling wx[HV]SCROLL in wxScrolled (see #17846).

Closes #14856, #17846.

include/wx/scrolwin.h

@@ -415,17 +415,7 @@ public:
         if ( !(style & (wxHSCROLL | wxVSCROLL)) )
             style |= wxHSCROLL | wxVSCROLL;
 
-#ifdef __WXOSX__
-        bool retval = T::Create(parent, winid, pos, size, style, name);
-        if ( retval && (style & wxALWAYS_SHOW_SB) )
-            ShowScrollbars(wxSHOW_SB_ALWAYS, wxSHOW_SB_ALWAYS);
-        return retval;
-#else
-        if ( style & wxALWAYS_SHOW_SB )
-            ShowScrollbars(wxSHOW_SB_ALWAYS, wxSHOW_SB_ALWAYS);
-
         return T::Create(parent, winid, pos, size, style, name);
-#endif
     }
 
 #ifdef __WXMSW__