Bug 1057038 - Temporary fix to disable completely audioLevel widget in TB.js

Note that this is a temporary fix until a more adequate solution is provided by TB. Merging this now because it has a huge impact in the app performance.
Could you leave a note with a reference to the bug in which we are changing this please? Something like:
// We set _audioLevelCapable to false on bug https://bugzilla.mozilla.org/show_bug.cgi?id=1057038
There is no need to left what it was because we will have the change in the PR attached to the bug.