remove Dashboard activity and related assets

All of the Dashboard functionality has been migrated to other parts of
the app:
 - menu drawer for navigation
 - application-wide Settings activity for adding and removing blogs, as
   well accessing the About screen
 - Posts activity for refreshing blog content

git-svn-id: https://android.svn.wordpress.org/trunk@937 a6565c4b-a94d-4d28-bb26-aa9e5ab542c0