gunixmounts: Prevent unwanted automount requests

mnt_table_is_fs_mounted causes unwanted automount requests due to
canonicalization of source and target. It might be replaced by
mnt_table_find_source as per the documentation in order to prevent
the automounts, but it is redundant. All mtab entries should be already
mounted and thus mnt_table_is_fs_mounted result is always true (it
basically checks that the fs from mtab is in mtab). Let's remove
the check at all.

https://bugzilla.gnome.org/show_bug.cgi?id=781867