mediawiki.legacy.common: don't embed legacy images

mediawiki.legacy.common: don't embed legacy images

This drops the /* @embed */ directive for the images referenced by the
mediawiki.legacy.common stylesheet. The images themselves and the styles which
apply them remain. They will simply not be pre-loaded as data URIs within the
stylesheet, which is fantastic because mediawiki.legacy.common is top-loaded on
all pages and embedded images make up nearly half of its weight. With this
change, the gzipped size of mediawiki.legacy.common drops from 27.6k to 15.6k.

Affected classes:

* `.mw-icon-question`: Used for tooltips generated by Preferences::getOptionFromUser.
* `.mw-help-field-hint`: Not in use since I8e092e9d.
* `.mw-small-spinner`: Not in use since I2a2556a0.
* `.mw-ajax-loader`: Not in use.

Change-Id: I6c7e592a502ae14b9f81165f10a1d4240e592649