Drop FTE related bits in Tor Browser

FTE is unmaintained and hardly used by Tor Browser users. Moreover, it is not available on all platforms/architectures due to a bunch of reasons (see legacy/trac#24195 (moved) for 64bit Windows and legacy/trac#18495 (closed) for macOS). Windows is going away entirely as well shortly with the transition to Debian Stretch (see: legacy/trac#29319 (moved) and legacy/trac#29307 (moved) for the rationale). We should drop the remaining bits in Tor Browser while we are moving to Tor Browser 9 and close legacy/trac#28521 (closed) as well.

I had some hope for getting Marionette included into Tor Browser 9 which is why I wrote a patch for getting it tested in our nightly builds (see: legacy/trac#29623 (moved)). However, it's not clear yet whether that happens or whether that would be a smart idea given its state, alas.