Fix "Call to undefined method ProofreadPageDifferenceEngine::getWarningMessageText()"

Stop the fatals.
Let the new difference engine take care of its warnings.

Bug: 64037
Change-Id: Ibe8e67ebfcbf7619c949c7b0ae29246837af9ab2
Merge "Fix "Call to undefined method ProofreadPageDifferenceEngine::getWarningMessageText()""