Remove crosswalk

[namenotrequired]

As discussed here: #1047

I removed the section about problems with web views in Android 4 and previous, and how crosswalk helps you solve them.

For those already using crosswalk, I decided to keep the crosswalk instructions in the "deploying to production" section. I only fixed the link.

TODO

• If this is a significant change, update CHANGELOG.md
• Use <h2 id="foo"> instead of ## Foo for headers
• Leave a blank line after each header

[lorensr]

Thanks! ☺️