Bug 1023407 - Revert switch-to-new-tab ButtonToast (Bug 997288) in Fennec <= 32. r=margaret,a=lsblakk

ButtonToasts interact badly with touch events, blocking interaction with the
rest of the system until the ButtonToast expires or is otherwise dismissed
(Bug 1019735). Until this is fixed, we'll just show system Toasts.

This patch is *not* a straight back-out; it is a selective disabling of the
new functionality.