gdsys: Post ppc4xx removal cleanup

The ppc4xx architecture was removed, and with it several old gdsys 44x
boards, but some "debris" from these purged boards was left over.

This patch removes these remnants (mostly entries in Makefiles, some now
superfluous data structures and some now obsolete config variables from
the whitelist).

Signed-off-by: Mario Six <mario.six@gdsys.cc>