Bug 1023087 - [Window Management] Marionette test for copy-paste

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
We don't need this line[1].
[1] https://github.com/mozilla-b2g/gaia/pull/20910/files#diff-1b27bbfeebcd899b3619e0bbf1767af3R49