target: remove unused "bitfield" infrastructure

target: remove unused "bitfield" infrastructure

We have too many different registers, and too many version and
context dependent interpretations, for this type of bitfield
management to be scalable.

(Anyone who really wants bitfield interpretation *can* do that
in Tcl code...)

There are ... quite a few copies of the same ARM dummy registers.
There should eventually be one copy; this many is craziness.

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