Requests for comment/Ditch crappy API formats

Here's the breakdown of API formats requested by users from Wikimedia sites during June 19, 2014: 375079448	json 34750831	xml 13068687	php 243625	xmlfm 71337	txt 62549	jsonfm 41158	text/x-wiki 34458	dbg 24460	yaml 7532	txtfm 34	yamlfm 31	dbgfm 28	none 25	rawfm 23	dump 22	phpfm 22	wddxfm 22	dumpfm 18	wddx 4	html 4	jsonf 3	jsonp 3	j 2	atom 2	raw 2	text 2	csv 2	PHP%20print_r 1	wdxfm 1	default 1	pjson 1	xhtml 1	XML 1	plain 1	htmlfm 1	yamfm 1	xml%27 1	xm%20l 1	js 1	jsonaction%3Dquery 1	json%0D%0A 1	xml%27,%200,%20true 1	xml%27,0,true 1	jaon 1	xmuslfm

Background
When API was first created, what it was supposed to be used for was unclear, so authors added several really weird output formats.

Problem
Some formats shouldn't have been added in the first place, now they're largely unused. They, however extract their toll on maintainers and general sanity of code base (WDDX to integrate with ColdFusion, really? PHP variable dumps that were never intended to be machine-readable, really?)

Proposal
Kill outright, with maximum cruelty:
 * dump
 * wddx

Deprecate slowly, with a clear timeline:
 * dbg
 * txt
 * yaml - alias to json, so updating callers should be trivial. Besides, most yaml calls seem to be from just 2 large third-party reusers.

Keep:
 * json
 * none - used internally, so not visible in the above stats
 * php - sigh
 * raw - used internally, so not visible in the above stats
 * xml - should really die a fiery death, but outside of this RFC