`conf/workers-shared-extra.yaml.j2` is applied to monolith Complement runs

This issue has been migrated from [#13590](https://github.com/matrix-org/synapse/issues/13590).

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[`docker/complement/conf/workers-shared-extra.yaml.j2`](https://github.com/matrix-org/synapse/blob/3dd175b628bab5638165f20de9eade36a4e88147/docker/complement/conf/workers-shared-extra.yaml.j2) seems to apply even when `SYNAPSE_COMPLEMENT_USE_WORKERS=true` isn't set. I assume they aren't supposed to stack like that for the monolith?

Noticed because I saw the `reject_limit` as `99999` when investigating https://github.com/matrix-org/synapse/pull/13541#discussion_r952064254. Disabling the rate limit could be as expected for the monolith but the way it's inherited from workers seems weird.

This is the command I was running for reference but it probably works in the simplified case as well,
```
TEST_ONLY_IGNORE_POETRY_LOCKFILE=1 TEST_ONLY_SKIP_DEP_HASH_VERIFICATION=1 COMPLEMENT_DEBUG=1 COMPLEMENT_KEEP_BLUEPRINTS="fed.perf_many_messages.hs1" COMPLEMENT_ALWAYS_PRINT_SERVER_LOGS=1 COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh -run TestMessagesOverFederation -p 1
```

### Dev notes

Related PRs:

- https://github.com/matrix-org/synapse/pull/12404
- https://github.com/matrix-org/synapse/pull/13279

Start with docker/complement/conf/workers-shared-extra.yaml.j2 and inspect how the SYNAPSE_COMPLEMENT_USE_WORKERS setting selects configuration for Complement runs. Reproduce the behavior with the listed scripts-dev/complement.sh command, then verify that monolith runs no longer inherit the workers-specific configuration while worker runs still do.

Written by the indexing model from the issue text.