Remove `columnPrefix = ""` from communal TableLogBuffers.

After I8fbe2b7dbf35f45ece10a23d5c20da3e5d7b7772, `columnPrefix` is
defaulted to the empty string so classes no longer need to manually
provide it.

Bug: N/A
Flag: EXEMPT refactor
Test: communal dumps still work
Change-Id: I2d8533f8765542a9123cc3e32d871daa46a3bf64