tile: fix "odd fault" warning for stack backtraces

We were setting ex1 in new kernel threads to KERNEL_PL.
But since we just do a simple context-switch, not an iret,
any value set here is ignored anyway, and its presence causes
stack backtraces to end with a warning about an "odd fault".
Signed-off-by: NChris Metcalf <cmetcalf@ezchip.com>

arch/tile/kernel/process.c

@@ -133,7 +133,6 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 		       (CALLEE_SAVED_REGS_COUNT - 2) * sizeof(unsigned long));
 		callee_regs[0] = sp;   /* r30 = function */
 		callee_regs[1] = arg;  /* r31 = arg */
-		childregs->ex1 = PL_ICS_EX1(KERNEL_PL, 0);
 		p->thread.pc = (unsigned long) ret_from_kernel_thread;
 		return 0;
 	}