arm64: Remove unused assembly macro

As of commit 4141c857

 ("arm64: convert
raw syscall invocation to C"), moving syscall handling from assembly to
C, the macro mask_nospec64 is no longer referenced.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>