Fix bad merge with start_background_tasks (#19013)

This was originally removed in
#18886 but it looks like it
snuck back in #18828 during a
[bad
merge](4cd3d91).

Noticed while looking at Synapse setup and startup (just by happen
stance).

I don't think this has adverse effects on Synapse actually working and
`start_background_tasks()` can be called multiple times.


### Is there a good way to audit all of these merges?

As I would like to see the conflicts for each merge.

This works but it's still hard to notice anything is wrong:

```
git log --remerge-diff <commit-sha>
```

> shows the difference from mechanical merge result and the result that
is actually recorded in a merge commit

via
https://stackoverflow.com/questions/15277708/how-do-you-see-show-a-git-merge-conflict-resolution-that-was-done-given-a-mer/71181334#71181334

The following better. Specify the version range to the commit right
before the merge to the merge. And can even specify which file to look
at to make it more obvious with the hindsight we have now.

```
git log --remerge-diff <merge-commit-sha>~1..<merge-commit-sha> -- synapse/server.py
```

Example:
```
git log --remerge-diff 4cd3d91~1..4cd3d91 -- synapse/server.py
```

Author: MadLittleMods

changelog.d/19013.misc

@@ -0,0 +1 @@
+Remove duplicate call to `hs.start_background_tasks()` introduced from a bad merge.

synapse/server.py

@@ -613,12 +613,6 @@ def setup(self) -> None:
         self.datastores = Databases(self.DATASTORE_CLASS, self)
         logger.info("Finished setting up.")
 
-        # Register background tasks required by this server. This must be done
-        # somewhat manually due to the background tasks not being registered
-        # unless handlers are instantiated.
-        if self.config.worker.run_background_tasks:
-            self.start_background_tasks()
-
     # def __del__(self) -> None:
     #    """
     #    Called when an the homeserver is garbage collected.