arch: powerpc: kvm: book3s_32_mmu.c: Remove unused function

Remove the function sr_nx() 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>
Signed-off-by: Alexander Graf <agraf@suse.de>