[AArch64] Remove references to Streaming SVE from target features.

Following discussion on D120261 and D121208 it seems better to remove the
concept of Streaming SVE from the subtarget/assembler predicates and
instead reason about 'SVE' and 'SME' as its higher level features, rather
than trying to model this runtime mode through explicit feature flags.

This patch is largely NFC.

Reviewed By: paulwalker-arm, david-arm

Differential Revision: https://reviews.llvm.org/D125977