remove lint-readme

[davidchambers]

Linting readmes involves significant complexity.

First, it requires four dependencies:

• eslint-plugin-markdown
• remark-cli
• remark-lint-no-undefined-references
• remark-lint-no-unused-definitions

Secondly, lint-readme itself is complex, especially the rewrite function that relies on heuristics to append semicolons.

Finally, each project that depends on sanctuary-scripts needs to configure ESLint appropriately, which can be messy.

The juice is not worth the squeeze.