remove useless and incorrect uc_regspace member from mips ucontext_t

this seems to have been copied erroneously from the arm version of the
file. it's fairly harmless but it's a mistake and better to fix now
than later.

arch/mips/bits/signal.h

@@ -44,7 +44,6 @@ typedef struct __ucontext {
 	stack_t uc_stack;
 	mcontext_t uc_mcontext;
 	sigset_t uc_sigmask;
-	unsigned long uc_regspace[128];
 } ucontext_t;
 
 #define SA_NOCLDSTOP  1