Remove unused Ada opcodes

Remove unused Ada opcodes

Several Ada expression opcodes are now unused, and can be removed.
Most of these are handled in a different way by the code.
OP_ATR_IMAGE, however, was never implemented.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* std-operator.def (OP_NAME, OP_ATR_IMAGE, OP_ATR_MODULUS)
	(OP_OTHERS, OP_CHOICES, OP_POSITIONAL, OP_DISCRETE_RANGE):
	Remove.