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
(cherry picked from commit b8f4ff6bc17154c3cbde2ae24301caab4f2d5871)