mwcore-docgen: drop the version logic

Zuul now generates a parameter DOC_SUBPATH out of the tag refspec or the
branch name.  That is suitable to be used as a version.

Thus generate the documentation to $TARGET_BASEDIR/php. The Jenkins job
uses the push-doc macro with docdest: 'mediawiki-core/$DOC_SUBPATH'.

https://gerrit.wikimedia.org/r/#/c/166034/

Change-Id: I1fd6d23917c2098dbdad37d9e718335616207f68