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