Remove unnecessary giant acquisition around panic in #ifdef DIAGNOSTIC

code.

# There is some question about whether this code is even relevant any
# longer (it dates back to prehistoric times, i.e. present in r1.1),
# especially on amd64.

Reviewed by:	jhb

sys/amd64/amd64/trap.c

@@ -745,10 +745,8 @@ syscall(struct trapframe *frame)
 
 #ifdef DIAGNOSTIC
 	if (ISPL(frame->tf_cs) != SEL_UPL) {
-		mtx_lock(&Giant);	/* try to stabilize the system XXX */
 		panic("syscall");
 		/* NOT REACHED */
-		mtx_unlock(&Giant);
 	}
 #endif
 

sys/i386/i386/trap.c

@@ -924,10 +924,8 @@ syscall(struct trapframe *frame)
 
 #ifdef DIAGNOSTIC
 	if (ISPL(frame->tf_cs) != SEL_UPL) {
-		mtx_lock(&Giant);	/* try to stabilize the system XXX */
 		panic("syscall");
 		/* NOT REACHED */
-		mtx_unlock(&Giant);
 	}
 #endif