featureflag: Remove `FetchSourceBranchQuarantined`
requested to merge 4698-featureflag-enable-use-of-quarantined-repository-in-fetchsourcebranch into master
The flag FetchSourceBranchQuarantined is rolled out, now we can delete
the flag and modify the code/tests to remove the previous logic.
Closes #4698 (closed)