Remove directories of baseline images that we don't maintain

Also renaming "base" as "base-macpro" to be more descriptive.
I will add a base-macpro/README file in a subsequent CL (I fear what might
happen if I try to do that in the same CL as the directory rename).
Review URL: http://codereview.appspot.com/5440104

git-svn-id: http://skia.googlecode.com/svn/trunk@2796 2bbb7eff-a529-9590-31e7-b0007b416f81