[Obvious][AArch64] Delete unused aarch64_simd_emit_pair_result_insn.

[Alan Lawrence]

Bootstrapped on aarch64-none-linux-gnu.

Pushed as r222234.

gcc/ChangeLog:

        * config/aarch64/aarch64.c (aarch64_simd_emit_pair_result_insn): Delete.
        * config/aarch64/aarch64-protos.h (aarch64_simd_emit_pair_result_insn):
        Delete.