Remove classy/stable base theme

Looks like a few more things need to be done here https://git.drupalcode.org/search?search=classy&nav_source=navbar&projec...

Here's an updated patch that removes the rest.

The ones that were used were:

classy/indented
classy/node
classy/search-results
classy/file

If we want any of those items, we could just create our own.