cells: remove conflict between CellAdapter and CellShell

The 'say' and 'exec context' commands exist both in CellAdapter and
CellShell.  This is normally not a problem unless an instance of
both classes share the same command context, which happens for the System
cell.

This patch removes the conflict by dropping the two commands from
CellAdapter.  The effect of doing this is to remove these two commands
from cells other than System.

Neither command is used by dCache outside of the System cell (or some
other CellShell), so dCache itself will not be affected by these changes.

It's possible, but I think unlikely, that our admins will use either
command.

FOR RELEASE NOTES

The 'say' and 'exec context' commands are no longer available from
cells other than the System cell.

Target: master
Requires-notes: yes
Requires-book: yes
Patch: http://rb.dcache.org/r/6523/
Acked-by: Gerd Behrmann