Bug 1350039 - Remove remaining Linux external media test. r=dustin

In bug 1347269 I missed removal of a test set that should have been removed.
This follow up removes this set also.

MozReview-Commit-ID: Be4GjLm6OgB

--HG--
extra : rebase_source : ee29ebbe98a26994cc9ba87920f2191cbe3c9c6f

taskcluster/ci/test/test-platforms.yml

@@ -19,7 +19,6 @@ linux32/debug:
     build-platform: linux/debug
     test-sets:
         - linux32-tests
-        - external-media-tests
 linux32/opt:
     build-platform: linux/opt
     test-sets: