[Arm][MC] Remove unused prefixes in .arch_extension fp tests

idiv: There is no difference between Armv7m and Thumbv7M
behaviour so the specific CHECKs are not needed.
The errors for Armv7-a and Thumbv7-a will always
include "ARM" or "THUMB" respectively so they need their
own CHECK prefix, making CHECK-V7 redundant.

mp: Behaviour is dependent on whether the triple is v6/v7/v7M
regardless of being Arm or Thumb. So we don't need the more
specific CHECK-ARMv7M etc.

simd: Errors are either v7 only, or v7 and v8 so CHECK-V8
is not needed.

fp: Same as simd

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D90918