Remove unused function release_get_by

Summary:
release_get_by has never been used except by its own tests, and it
still uses the legacy occurrence_* tables that we want to remove (T830).

Test Plan:

• make test in swh-environment passes
• grep -r release_get_by swh-* yields no result

Reviewers: Reviewers!

Differential Revision: https://forge.softwareheritage.org/D460