serial: xilinx_uartps: Remove leftover __exit_p()

__exit_p() need to be removed after the __devexit
removal from the driver.

Warning log:
drivers/tty/serial/xilinx_uartps.c:996:12:
warning: 'xuartps_remove' defined but not used [-Wunused-function]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>