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>

arch/arm64/include/asm/assembler.h

@@ -123,17 +123,6 @@ alternative_else
 alternative_endif
 	.endm
 
-/*
- * Sanitise a 64-bit bounded index wrt speculation, returning zero if out
- * of bounds.
- */
-	.macro	mask_nospec64, idx, limit, tmp
-	sub	\tmp, \idx, \limit
-	bic	\tmp, \tmp, \idx
-	and	\idx, \idx, \tmp, asr #63
-	csdb
-	.endm
-
 /*
  * NOP sequence
  */