aarch64: Fix sve2p1 dupq instruction operands.

This patch fixes the syntax of sve2p1 "dupq" instruction by modifying the way
2nd operand does the encoding and decoding using the [<imm>] value.

dupq makes use of already existing aarch64_ins_sve_index and aarch64_ext_sve_index
inserter and extractor functions. The definitions of aarch64_ins_sve_index_imm (inserter)
and aarch64_ext_sve_index_imm (extractor) is removed in this patch.

This issues was reported here:
 https://sourceware.org/pipermail/binutils/2024-February/132408.html