[GTK] Layout test editing/pasteboard/copy-standalone-image.html failing

[GTK] Layout test editing/pasteboard/copy-standalone-image.html failing
https://bugs.webkit.org/show_bug.cgi?id=163184

Unreviewed test gardening.

Remove obsolete failure expectation for copy-standalone-image.html.

The bug was fixed in r212428. (The test was also broken by both landings for
bug 170956, until r216174 fixed it again.)

Patch by Ms2ger <Ms2ger@igalia.com> on 2017-08-02

* platform/gtk/TestExpectations:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220127 268f45cc-cd09-0410-ab3c-d52691b4dbfc