Redacted media file is still accessible across bridges

[Mikaela]

I sent a media file in IRC-bridged Matrix room, redacted it and expected the file to disappear, but instead the link at IRC continues working.

I originally opened this issue at matrix-org/matrix-appservice-irc#473 where (or in the #irc:matrix.org) room I was told that it's a Synapse issue. I searched for the issues and found #1287 which was said to be related, but that this is a different issue.

[richvdh]

I think this is a dup of #1263