We include the file in plenty of places. This is mostly due to
historical reasons. The only place that needs something from the
header file is storage_backend_fs which opens _PATH_MOUNTED. But
it gets the file included indirectly via mntent.h. At no other
place in our code we need _PATH_.*. Drop the include and
configure check then.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>