This patch also removes the 'extensions.torbutton.no_tor_plugin'
pref and simply uses the 'plugin.disable' pref instead.

Thanks to our patch for Bug 10280 in tor-browser.git,
buttons on the plugins section of about:addons
already let you "Enable plugins" and "Disable plugins"
(i.e., toggle the 'plugin.disable' pref).

Also it's probably better to remove this footgun in the UI.