Get rid of ORMTable::getFieldPrefix

See I46a0d7e.

Change-Id: Ica8b7a8e012adc73ed4d8cc84129e7c6c95e76c3
(cherry picked from commit f856ec852eb4fef3b6ecce202e8cd9530b68f9ff)
Merge "Get rid of ORMTable::getFieldPrefix" into wmf/1.25wmf4