fix repository deletion by ensuring discussion threads/comments are also removed
Created by: slimsag
Otherwise we will violate DB constraints.
Fixes https://github.com/sourcegraph/enterprise/issues/13413
Note: This does not handle a more general issue of us not using transactions (both throughout this deletion code and the code discussions code in general.)
This PR does not need to update the CHANGELOG because it was not broken in the prior release.