Remove deprecation trigger at MigrateTools::addToSyncSourceIds in 7.x

Problem/Motivation

In #3104268: Sync is too strict during id comparison and can roll back everything a deprecation warning was added for 6.x

Steps to reproduce

Check MigrateTools::addToSyncSourceIds

Proposed resolution

Remove deprecation warning in 7.x

Remaining tasks

1. Wait for 7.x branch to change to 'Active' status
2. Remove deprecation

User interface changes

N/A

API changes

N/A

Data model changes

N/A