arm64: compat: Remove incorrect comment in compat_siginfo

The comment was right originally but the _pad array size was wrong. It
was fixed in the meantime but the comment not updated.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/compat.h

@@ -161,7 +161,6 @@ typedef struct compat_siginfo {
 	int si_code;
 
 	union {
-		/* The padding is the same size as AArch64. */
 		int _pad[128/sizeof(int) - 3];
 
 		/* kill() */