aarch64: Remove incorrect disassembly constraint

aarch64: Remove incorrect disassembly constraint

A check in print_insn_aarch64_word asserted that part of the encoding
space couldn't contain any valid encodings, and then returned ERR_NYI
("Not Yet Implemented", perhaps?) for these values.  However, some of
the new FEAT_MOP4 instructions will trigger the assert.  The check seems
to be outdated, and is clearly no longer valid, so it can just be
deleted.

Additionally, there are no other assignments of ERR_NYI, so delete all
remaining references to this error type.