Revert r51128, and its follow-up revisions r51129, r51130, r51390.

Revert r51128, and its follow-up revisions r51129, r51130, r51390.
Commit message was: "* (bug 18958) Added ability to disable entire variant conversion engine per user preferences (languages with language converter class only)"
Reverted because of multiple issues on CodeReview, notably:
* Unexplained rename of a configuration variable, with no backwards-compatibility code.
* Suspected parser cache pollution.
* Variant tabs flip-flop with parser cache misses and hits.
* Hacky implementation: changing configuration variables on the fly is almost always a bad idea, unless you are writing a configuration extension.
* Implementation of default is done as a special case in code accessing the preference, rather than by adding an entry to $wgDefaultUserOptions.