Remove unnecessary I420 VideoFrame creation
In makeI420Frames, there is a VideoFrame created without meaningful
YUV data. This redundant creation should be removed.
Differential Revision: https://phabricator.services.mozilla.com/D215159
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1902115
gecko-commit: 2da61495720af395f4856173050dc78a5e361ba2
gecko-reviewers: media-playback-reviewers, padenot