[CommCoreModule] Introduce CommCoreModule::clearNotifyToken

Summary: Clears notifyToken from the SQLite metadata table.

Test Plan:

1. Add notifyToken entry to the metadata table
2. Call global.CommCoreModule.clearNotifyToken()
3. Ensure that the entry has been deleted: https://blob.sh/atul/709c.png

Reviewers: def-au1t, palys-swm, varun, karol-bisztyga, ashoat, jimpo

Reviewed By: palys-swm, ashoat

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3172