[tor-project] Remove mercurius4 obfs4 bridge from Tor Browser?

[David Fifield]

According to my and Lynn Tsai's bridge reachability measurements, the
default obfs4 bridge mercurius4 has not been running for a few months.

It was refusing connections in March and April, and more recently has
just been timing out. Time to remove it?

pref("extensions.torlauncher.default_bridge.obfs4.6", "obfs4 104.131.108.182:56880 EF577C30B9F788B0E1801CF7E433B3B77792B77A cert=0SFhfDQrKjUJP8Qq6wrwSICEPf3Vl/nJRsYxWbg3QRoSqhl2EB78MPS2lQxbXY4EW1wwXA iat-mode=0");