Removed rules in documentation

The following rule pages respond with 404 when access from the rule list. ESlint CLI doesn't recognized these either:

• default-case-last
• no-useless-backreference
• no-restricted-exports

[mdjermanovic]

Sorry about that! These are actually new rules that will be available in v7.0.0:

• https://eslint.org/docs/7.0.0/rules/default-case-last
• https://eslint.org/docs/7.0.0/rules/no-useless-backreference
• https://eslint.org/docs/7.0.0/rules/no-restricted-exports

You can install v7.0.0-alpha.2 and try out the rules with: npm i eslint@next --save-dev

These rules shouldn't appear in the list for the actual version, and that's a bug we should fix.

I'm closing this as a duplicate of #688