Remove Levels copied from learngitbranching.js.org

[benthayer]

By the time we release, we'll have replaced all of the following levels with entirely our own content.

1. Basics (basics)
├── 1. Introduction to Commits (committing)
├── 2. Branching in Git (branching)
├── 3. Merging in Git (merging)
└── 4. Introduction to Rebasing (rebasing)
3. Rework (rework)
├── 1. Cherrypicking (cherrypicking)
└── 2. Interactive Rebase (irebase)
4. Mixed Bag (mixedbag)
└── 1. One Commit (onecommit)

To ensure all of the content is our own for the 1.0 release, we should remove these as well

2. Rampup (rampup)
├── 1. Detaching HEAD (detaching)
├── 2. Relative References I: Using (^) (relrefs1)
├── 3. Relative References II: Using (~) (relrefs2)
└── 4. Reversing Changes in Git (reversing)

[benthayer]

I will do this right before the v1.0 release