arm64: insn: remove aarch64_insn_gen_prefetch()

arm64: insn: remove aarch64_insn_gen_prefetch()

There are no users of aarch64_insn_gen_prefetch(), and which encodes a
PRFM (immediate) with a hard-coded offset of 0.

Remove it for now; we can always restore it with tests if we need it in
future.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20221114135928.3000571-2-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>