settings: drop the dom.workers.sharedWorkers.enabled pref for TB 6.0
settings: drop the dom.workers.sharedWorkers.enabled pref for TB 6.0

The pref has been removed from firefox:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1207635

boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite,boklm/tbb-testsuite