x86: merge sys_rt_sigreturn between 32 and 64 bits

Impact: cleanup

With the recent changes in the 32-bit code to make system calls which
use struct pt_regs take a pointer, sys_rt_sigreturn() have become
identical between 32 and 64 bits, and both are empty wrappers around
do_rt_sigreturn().  Remove both wrappers and rename both to
sys_rt_sigreturn().

Cc: Brian Gerst <brgerst@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>