Don't use XRegExp for Ovid Tagged

As far as I can tell was the only use of XRegExp outside of an export
translator. This means that we won't need to load XRegExp for anything
but export.

I'd eventually like to kill XRegExp entirely because:

- It's extra code that is not used in many places.
- Sandboxing it is disproportionately painful because it creates new
  object instances. (Every web translator now works in Firefox 33, but
  I'm not sure if XRegExp is entirely broken or it was just line 229
  here.)
- ES6 Unicode regexp support should eventually make it unnecessary.
Merge pull request #786 from zotero/sjk/ovid

Don't use XRegExp for Ovid Tagged