Revert "Use mw-wiki-logo class instead of inline background-image"

Caused bug 71334.

This reverts commit 9ebb220b7f72d3702f459fb597f360f5e24c9f0c.

Bug: 71334
Change-Id: I19736c98d5f66319c1cb1425a28cd8acd4fd364e
(cherry picked from commit b74bb7ae9abb60db2d47e6bb9bc332493ae6128e)