phpunit: Remove unused assertNotTag() and assertTag() overrides

phpunit: Remove unused assertNotTag() and assertTag() overrides

Deprecated since PHPUnit 4.2, we use PHPUnit 4.8 and would like
to be compatible with PHPUnit 5 which no longer has this method.

The current hack was only enough to avoid the deprecation warning,
the underlying logic in Xml::findNodes() was also removed in PHPUnit 5.

Follows-up 23ec64745c, dba0522f70, T71505.

* assertNotTag() is not used anywhere in Wikimedia Git.
* assertTag() was used in 1 test in core, fixed in the parent commit.
  Match in mediawiki/skins/chameleon is unrelated.

Change-Id: I56b8f1f88124d69190938fc522ea75419e96c49c