consolidate declarations of k_sigaction

Only alpha and sparc are unusual - they have ka_restorer in it.
And nobody needs that exposed to userland.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/mips/include/uapi/asm/signal.h

@@ -102,10 +102,6 @@ struct sigaction {
 	sigset_t	sa_mask;
 };
 
-struct k_sigaction {
-	struct sigaction sa;
-};
-
 /* IRIX compatible stack_t  */
 typedef struct sigaltstack {
 	void __user *ss_sp;