gas/NEWS: drop mention of Arm64's SVE2.1 and SME2.1

... plus the SME part of B16B16. As per

https://sourceware.org/pipermail/binutils/2024-February/132408.html

SVE2.1 support is both incomplete and buggy. SME2.1 "support" goes as
far as a single instruction (a subset of movaz forms) only. The SME part
of B16B16 is entirely missing.