Removed SkinBlueSky::showHeadSection(), it was unused.

Two wikiHow extensions, DocViewer and Quizzes, still hook into the ShowHeadSection hook, but the showHeadSection() function was only used in the previous iteration (version 3) of the wikiHow skin, which was superseded by the version that BlueSky is based on in autumn 2013.

Change-Id: If3476c83007f7c86761cf72bd34341391993efe5