Revert "Bug 10298: Provide a preference to enable mixed content rules."

https://trac.torproject.org/projects/tor/ticket/10344

This reverts commit 613ea2f465b558bcf0343933e3dfe0a8568c37fa.
When toggling ruleset state, don't load the library twice
Merge branch '3.0' of https://git.torproject.org/https-everywhere into 3.0
Update changelog