resourceloader: Remove wfExpandUrl() from ResourceLoaderImage

resourceloader: Remove wfExpandUrl() from ResourceLoaderImage

Follows-up b5e69c4ef which did the same for already for ResourceLoader.php.

There's no need to expand the hostname into these urls. If load.php
is on the same domain (e.g. set to '/w/load.php') then this can be
resolved by the browser normally.

If load.php is on a custom domain, the stylesheet would be served
from there instead of from the wiki domain. However in that case
the hostname would already be part of $script value ($wgLoadScript).

Bug: T125292
Change-Id: I7242445335d69d7ae290da5f321a59edd537d819