Remove support for wxMotif

This port hasn't been updated for ages and is not used by anybody any
longer, so remove its code to facilitate maintenance.

Also remove references to this port from the documentation and most of
the other places (VMS-specific descrip.vms files still check for it
because it's not clear how to update them all), including configure.

Regenerate the latter and rebake all makefiles.

Finally document that this port is not available any longer.

.github/workflows/ci.yml

@@ -108,11 +108,6 @@ jobs:
             configure_flags: --with-directfb --enable-pch --disable-stc
             skip_samples: true
             allow_warnings: true
-          - name: Ubuntu 18.04 wxMotif
-            runner: ubuntu-latest
-            container: ubuntu:18.04
-            configure_flags: --with-motif --enable-pch --disable-stc
-            skip_samples: true
           - name: Ubuntu 18.04 wxQt
             runner: ubuntu-latest
             container: ubuntu:18.04