[AArch64][SME] Remove term 'streaming-sve' from assembler diagnostics.

'streaming-sve' is not a feature that users should be able to set,
hence why it shouldn't show up in user-diagnostics. The only
flag that end-users should be able to set is '+sme'.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D120256