capabilities: remove cap_mmap_file()

The cap_mmap_file() LSM callback returns the default value for the
security_mmap_file() LSM hook and can be safely removed.

Signed-off-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Serge Hallyn <sergeh@kernel.org>