board: gdsys: Remove commands to reduce footprint

Commit "2842c1c2 fit: add sha256 support" badly increased
memory footprint, so some of our boards did not build anymore.
Since monitor base must not be changed I removed some commands
to save memory.

Maybe making sha256 optional for fit would be an option for
the future since it really has some beefy footprint.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>