mediawiki.legacy.ajax: Pass JSHint

mediawiki.legacy.ajax: Pass JSHint

* Added closure so that "mw.util.wikiScript" doesn't access
  global "mw" directory but uses local reference to global
  "mediaWiki".
* Keps names as-is, as such disabled camelcase rule locally
  for this file. This code will be wrap-deprecated and removed
  over the next 2 releases.
* Turned into local cross-references and exposed those, that
  way the individual methods don't all call implied globals but
  are ensured to be referring to the original methods as they
  were defined in the same scope.
* Fixed clashing 'e' variable name.
* Removed unexpected "delete x" statement. Does nothing and
  causes a lint warning.
* Replaced "delete request" with "request = undefined". The
  purpose of the statement is to explicitly dereference the
  XHR object in older IE browsers. However "delete" is not
  allowed for local varables, setting to undefined has the
  same effect.

* Moved debug_mode and request_type down to the other exposures.

Change-Id: Ib45ed9bd8c6f4ebdb060a527a63eaf59d2839493