Remove 'cd' Overrides

There were multiple places in the repo where we were overriding the builtin `cd` command with a function. This is bad because

1. Each time someone creates a `cd` function it clobbers the one before it, so if you used the plugins that did this you would only really have access to the last one.

2. Zsh already has a hook for this.

I've included 3 different commits, each one removing a cd override from a different place. I'll use the github code commenting feature in addition to my commit messages to explain some of the changes I made.