Remove broken parsing of @media rules

This code didn't even pay attention to the
cur_stmt->kind.media_rule->media_list, and unconditonally considered
each statement in the ->ruleset to be of kind ruleset.  That seems
broken.

(The theme doesn't use any @media queries, and they are unsupported
anyway.)

Fixes GNOME/gnome-shell#1979
6 jobs for !876 with remove-media-rules in 3 minutes and 54 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #518563
check_commit_log

00:00:22

passed #518565
eslint

00:01:03

passed #518564
js_check

00:00:33

 
  Build
passed #518566
build

00:01:58

 
  Test
passed #518567
test

00:00:51

passed #518568
test-pot

00:00:49