Reduce repo size

[mikeharder]

Here is the clone size of the JS repo under four scenarios:

1. Full history of gh-pages: 2.16 GB
2. Squashed gh-pages: 83 MB
3. Delete gh-pages: 43 MB
4. Delete all branches except master: 40 MB

Squashing the branch is by far the most important, but even squashed the branch accounts for 50% of the total JS repo clone size.

All other branches combined add less than 10% to the repo size, so while it would be good to clean them up to remove clutter, they shouldn't have much perf impact.

[mikeharder]

Closing in favor of:

• Squash commits in branch gh-pages (Squash commits in branch gh-pages #4739)
• Investigating moving GitHub Pages content out of main repos (Investigating moving GitHub Pages content out of main repos azure-sdk#572)