arm64: Removed unused variable in compat_setup_rt_frame()

Recent clean-up of the compat signal code left an unused 'stack'
variable.
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/signal32.c

@@ -549,7 +549,6 @@ int compat_setup_rt_frame(int usig, struct k_sigaction *ka, siginfo_t *info,
 			  sigset_t *set, struct pt_regs *regs)
 {
 	struct compat_rt_sigframe __user *frame;
-	compat_stack_t stack;
 	int err = 0;
 
 	frame = compat_get_sigframe(ka, regs, sizeof(*frame));