imagescaling: remove legacy option for HHVM imagescalers

On ubuntu trusty, a newer version of librsvg is used, which supports by
default a security model where external entities are loaded only from
the current working directory.

Our SVG conversion code takes this into account

https://phabricator.wikimedia.org/diffusion/MW/browse/master/includes/media/SVG.php;97c4e93eefc7182fb4a166e752d44e8853fe3218$202

so we can make the arguments of rsvg-convert conditional on the presence
of HHVM.

Bug: T84842
Change-Id: I5d8e8a196bcab6a394af98191332b88dd09f6526