Refactor: libcrmcommon: Drop redundant check from mark_child_deleted()

mark_child_deleted() has only one caller (mark_xml_changes()). By the
time we reach this point, we can be sure that match_xml(new_parent,
old_child) will return NULL, because we checked it in the caller.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>