Remove markdownlint maximum line length rule

[dlipovetsky]

We currently have a 120 character maximum line length rule.

While maximum line length make sense for a programming language, I think it makes less sense for prose:

• The docs are written in a text editor, and almost any editor can automatically wrap lines. I don't want to manually wrap lines the first time, or re-wrap lines as I update prose.
• The docs are read in a browser and a text editor. Almost any browser or editor can automatically wrap lines. Manual wrapping can make it more difficult to wrap text automatically for a given viewport size.

I'd like to:

• Remove the rule
• Remove manually-inserted newlines from existing docs