staging: sm750: remove incorrect __exit annotation

The lynxfb_pci_remove function is used as the 'remove' callback
of the driver, and must not be discarded:

lynxfb_pci_remove' referenced in section `.data' of drivers/built-in.o: defined in discarded section `.exit.text' of drivers/built-in.o

This removes the extraneous annotation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>