Define __WXGTK20__ again for wxGTK 3 builds

This was accidentally broken in fb4f0b590c (Remove wxGTK1 port and GPE
support, 2022-10-16): even though the commit message explicitly said
that __WXGTK20__ was still defined, it actually wasn't any more.

Fix this now by just doing it always when using wxGTK. This allows to
drop WXGTK2 variable entirely, as it's not used any longer and we don't
need to set it.

Closes #23444.

configure

@@ -26696,7 +26696,6 @@ if test "$wxUSE_GUI" = "yes"; then
 
     GUI_TK_LIBRARY=
 
-    WXGTK2=
     WXGTK3=
     WXGTK4=
 
@@ -27686,8 +27685,7 @@ $as_echo "$wx_cv_lib_gtk" >&6; }
             3)      WXGTK3=1
                     TOOLKIT_VERSION=3
                     ;;
-            2.0)    WXGTK2=1
-                    TOOLKIT_VERSION=2
+            2.0)    TOOLKIT_VERSION=2
                     ;;
             *)      as_fn_error $? "
 The development files for GTK+ were not found. Please
@@ -35454,12 +35452,10 @@ _ACEOF
 _ACEOF
 
     fi
-    if test "$WXGTK2" = 1 ; then
-      cat >>confdefs.h <<_ACEOF
+            cat >>confdefs.h <<_ACEOF
 #define __WXGTK20__ 1
 _ACEOF
 
-    fi
 fi
 
 if test "$WXQT" = 1 ; then