This was a private mechanism to snapshot registers before leaving
debug state, and then on reentry to optionally display what changed.
It was coupled to the private register cache, which won't be sticking
around in that form for much longer.  Remove (instead of teaching
it how to handle *all* the registers).

(The idea is interesting, but we ought to be able to implement
this in a generic way.  Ideally through Tcl scripts that can
automatically be invoked following debug entry...)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>