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

This was partially found by using a static code analysis program called cppcheck.
Signed-off-by: NRickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
[PM: rewrite the patch subject line]
Signed-off-by: NPaul Moore <pmoore@redhat.com>