Remove the function pci_enable_swapping() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Patchwork: https://patchwork.linux-mips.org/patch/8867/Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org