libmount: (monitor) don't check for regular mtab

libmount: (monitor) don't check for regular mtab

The monitor supports utab only (as documented). It's application
responsibility to use libmount in the right way. It's overkill to
check for valid environment during monitor initialization.

For example systemd checks for regular mtab during boot, it's better
than try to be smart later in libmount monitor when system is already
running.

Signed-off-by: Karel Zak <kzak@redhat.com>