[mlir][ArmSVE] Cleanup dialect registration

ArmSVE dialect is behind the recent changes in how the Vector dialect
interacts with backend vector dialects and the MLIR -> LLVM IR
translation module. This patch cleans up ArmSVE initialization within
Vector and removes the need for an LLVMArmSVE dialect.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D100171