x86: Remove __mmask intrinsics in strstr-avx512.c

The intrinsics are not available before GCC7 and using standard
operators generates code of equivalent or better quality.

Removed:
    _cvtmask64_u64
    _kshiftri_mask64
    _kand_mask64

Geometric Mean of 5 Runs of Full Benchmark Suite New / Old: 0.958