Arm64/gas: drop redundant feature prereqs

Logic exists to deal with prereqs or prereqs, and in many cases
transitive prereqs are already not spelled out explicitly. Drop further
ones:
- FP is already a prereq to F16,
- SIMD and F16 are already prereqs to COMPNUM, and
- SVE2 and BFLOAT16 are already prereqs to SME.