[services] Tunnelbroker - Remove ToDeviceID index from messages table

Summary:
Because we've switched to the composite key in the messages table we don't need an additional index for a toDeviceID. We can use a hash_key.

Linear task: ENG-1493

Test Plan: Run yarn init-local-cloud is expected to be successful.

Reviewers: karol, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4695