selinux: drop unused macro

The macro _DEBUG_HASHES is nowhere used. The configuration DEBUG_HASHES
enables debugging of the SELinux hash tables, but the with an underscore
prefixed macro definition has no direct impact or any documentation.

Reported by clang [-Wunused-macros]

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>