Refactor: libcrmcommon: Drop pcmk_free_xml_subtree() internally

It's only two lines and not Pacemaker-specific; this is just the
standard way to free an XML node that may be part of a document.

Behavior is preserved in all three callers. However, for posterity:

• For free_xml_with_position(), we've already done any other checks that are necessary.
• For pcmk__strip_xml_text(), a text node can't be the document root, so we don't have to worry about freeing the document. ACLs and change tracking aren't relevant for the current callers; stripping text is done only internally and is essentially an implementation detail.
• For pcmk__apply_creation_acl(), we don't want to recheck ACLs (which would fail). Change tracking is irrelevant because the newly created node hasn't been accepted to the CIB yet.

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