Remove MySQL integration and dedicated code

[EvaBardou]

Refs #7811

[EvaBardou]

There are still a few places where mysql is mentioned in treeherder code, some changes might require a bit of investigation and development and should probably be applied as follow-ups:

• treeherder/bin/pre_deploy

  Line 24 in d2388c1

  # TODO: Look into this again when using newer MySQL and Django 2.x.

• treeherder/treeherder/etl/bugzilla.py

  Line 232 in d2388c1

  # distinct('bug_id') is not supported by Django + MySQL 5.7

• treeherder/treeherder/log_parser/failureline.py

  Line 61 in d2388c1

  # Sometimes get an error if we can't save a string as MySQL pseudo-UTF8

• https://github.com/mozilla/treeherder/blob/master/treeherder/perf/alerts.py#L147

• treeherder/treeherder/utils/queryset.py

  Line 13 in d2388c1

  can't make use of this while using MySQL which doesn't support streaming

• treeherder/treeherder/webapp/api/performance_data.py

  Line 360 in d2388c1

  # Django's distinct(*fields) isn't supported for MySQL

---

In the accessing_data.md and administration.md documentation files, the instructions to use "Treeherder's read-only MySQL replica" should also be reworked to apply to a Postgres replica instead.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.04%. Comparing base (704390f) to head (517b97c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7814      +/-   ##
==========================================
- Coverage   77.12%   77.04%   -0.09%     
==========================================
  Files         545      545              
  Lines       26989    26920      -69     
  Branches     3385     3393       +8     
==========================================
- Hits        20815    20740      -75     
- Misses       6007     6013       +6     
  Partials      167      167              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[EvaBardou]

This PR has been rebased on latest changes from master, this comment about MySQL's remaining mentions is still true and up to date.

[La0]

OK for me, but I'd like @Archaeopteryx input on the remaining few Mysql mentions (especially around .distinct() that should be usable now)

[Archaeopteryx]

Sorry for not landing this yet. I want to get the local development with remote PostgreSQL instance working which fails because the dockerflow package tries to apply migrations. I'll try to ifdef the package for dev environments.

[Archaeopteryx]

Thank you for the patch, the remaining mentions of MySQL in code comments as reported above will be addressed in follow-up commits, e.g. together with changes to use a remote GCP PostgreSQL instance locally.