mediawiki.searchSuggest: Code clean up

* Cache mw.Api instance.
* Remove redundant "str.length" check in fetch(),
  jquery.suggestions already takes care of this (it never calls
  fetch if the string is empty, and if it did, then this code would've
  left it unhandled causing bugs (e.g. suggestions of the previous
  fetch remain visible).
* Remove redundant promise.abort check.
* Use $.data() and $.removeData() to bypass unneeded convenience
  logic for collections and HTML data attributes.

Change-Id: I59ae21c4b481ef52e4db14cb1f3f2fe026c23b3f
Merge "mediawiki.searchSuggest: Code clean up"