Remove default redis config, which can cause permanent WSOD on pantheon

[wxactly]

Proposal to delete this default code. Open to other ideas though! I expected to be able to enable Redis on the Pantheon dashboard without any consequences, but here's what happened on my site:

1. I enabled Redis on the pantheon dashboard
2. All sites started showing WSOD
3. I panicked
4. I disabled Redis on the pantheon dashboard
5. All sites continued to show WSOD

I believe this is because Pantheon leaves the $conf['redis_client_host'] setting even after Redis has been disabled, but I honestly couldn't confirm that while panicking...

[mariacha]

This happens when the Redis module isn't installed or enabled on the site. It's definitely dangerous to have in for the reason you've described. I'll take it out, but I wish there were a way to check to see if the module is enabled before pulling in this code, but I haven't seen an answer to that.

But it's probably better to have Redis turned on and not working than it is to have Redis not turned on and a WSOD, so I'll turn this into a backlog ticket and think about it again.