mediawiki.action.history.diff: Remove white background from table.diff

Skins where a white background works generally already have a white background
behind this. Skins where it doesn't work generally just wind up overriding this
back to the default (transparent).

Simply removing the background should simplify things.

Change-Id: I1a906f533f7ae2697ee18c998f316a0957ac8bda
Merge "mediawiki.action.history.diff: Remove white background from table.diff"