Remove more HTTP pipelining preferences

HTTP pipelining is dead, so we removed a couple of preferences we set in #26603 (moved). But it seems we forgot some (thanks to https://blog.torproject.org/comment/276423#comment-276423 for the report)

network.http.pipelining.max-optimistic-requests
network.http.pipelining.read-timeout
network.http.pipelining.reschedule-timeout