whack-a-mole: no need to set_fs(USER_DS) in {start,flush}_thread()

flush_old_exec() has already done that.  Back on 2011 a bunch of
instances like that had been kicked out, but that hadn't taken
care of then-out-of-tree architectures, obviously, and they served
as reinfection vector...

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

arch/c6x/kernel/process.c

@@ -101,7 +101,6 @@ void start_thread(struct pt_regs *regs, unsigned int pc, unsigned long usp)
 	 */
 	usp -= 8;
 
-	set_fs(USER_DS);
 	regs->pc  = pc;
 	regs->sp  = usp;
 	regs->tsr |= 0x40; /* set user mode */