[simd.nonmembers] [simd.mask.nonmembers] Redundancies in subclause headings

[jensmaurer]

The subclauses under [simd.nonmembers] and [simd.mask.nonmembers] broadly violate the principle that a subclause heading shall not repeat (in part) a heading of a superordinate subclause.

See https://github.com/cplusplus/draft/wiki/Specification-Style-Guidelines#subclause-headings

See cplusplus/nbballot#875