Refactor: various: Drop pcmk__xe_add_node()

All but one caller passes 0 for nodeid. In those cases, it's equivalent
to a single crm_xml_add() call. For the caller that passed nodeid >= 0,
we simply check whether it's > 0 and call crm_xml_add() again if so.

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