Remove retired pluggable transports from BridgeDB
BridgeDB still hands out obfs3, ScrambleSuit, and FTE bridges. Tor Browser no longer supports FTE (see #29319 (moved)), so we should remove it. I suggest also removing obfs3 and ScrambleSuit because these transports don't offer anything that obfs4 doesn't already provide.
As of today, BridgeDB knows about 1,316 bridges. Among these:
31 support FTE. 29 of these wouldn't be handed out because they also support obfs4 (see #28655 (moved)). The remaining two bridges run FTE/obfs3 and ScrambleSuit/obfs3/FTE.
34 support ScrambleSuit. 32 of these also support obfs4 and only two don't. Instead, they run obfs3/ScrambleSuit and ScrambleSuit/obfs3/FTE.
106 support obfs3. Only seven of these don't support obfs4.
Considering the above, I think it's safe to retire FTE, ScrambleSuit, and obfs3.