Bug 1748221 - Disable specializing video on late beta and release. r=mstange

There are a number of pretty bad regressions caused by specializing
video (bug 1747999, bug 1738926, bug 1746711). It seems like it's better
to disable until we better understand the causes and scope of the
regressions.

Differential Revision: https://phabricator.services.mozilla.com/D134947