Use WatchedItemStore clearing in SpecialEditWatchlist

Use WatchedItemStore clearing in SpecialEditWatchlist

This switches to using the WatchedItemStore that has the ability to
delete all watchlist items for a user using the jobqueue.

The job was introduced in:
Icea573a10078ea3f09dc2e4e9fdc737bf639935d
Functionality added to the store in:
I2f8c8be860a03116535dbd1e6a18ce2e8bc546b4
This patch is the first code calling the new functionality.

If the number of watched items for the user is less
than the number that can be completed in a single
batch then they will still be deleted during the
interactive session.

Bug: T68212
Bug: T129481
Change-Id: I2f8c8be860a03116535dbd1e6a18ce2e8bc546b4