Implement utility method to remove notifications olm account storage

Summary:
This differential implements utility method to remove strage of notifications olm account. We need this functionality since public olm account is removed every loggout since it is stored in SQLite. I think we should follow similar semantics here. Implementation of this method
ensures we are not throwing an error if the file does not exist (attempt to delete prior to proper creation).

Test Plan:
Use test plan from parent differential and add call to this method after call to method that retrieves public keys. Run app one more time and ensure that keys returned from getNotificationsIdentityKeys are different (they were deleted on previous run, so thay are created
new on the next run).

Reviewers: tomek, bartek, kamil

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6779