Don't overlay the Material.Light theme's homeAsUpIndicator attribute

Current API expects the default ActionBar styles not to have the
homeAsUpIndicator defined, thereby inheriting it from the theme itself
when changed there. Remove the stray definition in the light material
action bar style that was blocking this from happening.

Bug 15458606

Change-Id: Ia7fdbde814582a1d3eb8743bf76bc019f6ac7488