[keyserver] Remove UPDATE_RELATIONSHIP.farcaster_mutual messages from KeyserverDB

Summary:
Similar to D11669 and D11680, but for keyserverDB.

Depends on D11680

Test Plan:
Before migration:

After migration:

Change in number of results for SELECT * from messages:

Reviewers: ashoat, ginsu, tomek, varun, will

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D11681