Remove incorrect migration file from state logical DB (#13788)

* Remove incorrect migration file from `state` logical DB

The table `ex_outlier_stream` is part of the `main` logical DB; it
should not have been created in the `state` logical DB. We remove this
migration now as a tidy-up.

Note: we cannot `DROP TABLE IF EXISTS ex_outlier_stream` in a new
migration, because some (most) instances of Synapse host both of these
logical DBs on the same DB cluster.

* Changelog

Author: David Robertson

changelog.d/13788.misc

@@ -0,0 +1 @@
+Remove an old, incorrect migration file.