Remove crosswalk

[namenotrequired]

As discussed here: https://github.com/meteor/guide/issues/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

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

[lorensr]

Thanks! ☺️