Delete event_push_summary_unique_index again. (#14669)

if a Synapse deployment upgraded (from < 1.62.0 to >= 1.70.0) then it
is possible for schema deltas to run before background updates causing
drift in the database schema due to:

1. A delta registered a background update to create an index.
2. A delta dropped the above index if it exists (but it yet exist won't since
  the background job hasn't run).
3. The code assumed the index was dropped.

To fix this we:

1. Cancel the background update which could create the index.
2. Drop the index again.
3. Drop a related index which is dropped by the background update.