Web customizer still shows deprecated @screen-xs and @screen-phone

[fulldecent]

Page is: http://getbootstrap.com/customize/#media-queries-breakpoints

These variables are deprecated since 3.0.1 per https://github.com/twbs/bootstrap/blob/master/less/variables.less#L263-L268

(Original note from @brejoc on stackexchange https://stackoverflow.com/questions/18424798/twitter-bootstrap-3-how-to-use-media-queries/18850777?noredirect=1#comment35328440_18850777)

[cvrebert]

Actually, even more of the variables in that section are deprecated, but removing them while still preserving customizability would require additional postprocessing...

We could just add customizer-visible notes (via //**) that these variables are deprecated. @mdo How's that sound?

[mdo]

@cvrebert Sounds fine.