Phase out $wgRateLimitLog in favor of debug bucket

I have deprecated $wgRateLimitLog in MediaWiki (since Feb 2014) in favor
of the 'ratelimit' debug log bucket.  The code has been made to remain
back compatible, this change is merely a cleanup change.  The
$wgRateLimitLog value is already assigned to the 'ratelimit' bucket by
MediaWiki.

MediaWiki core change is:
https://gerrit.wikimedia.org/r/115902 (commit 5b52c881b).

Change-Id: I42406582784c671d88b1849416671328010eee0d