Remove topics-todo and references to it

Effectively all those topics are now curated, so it's no longer
helpful to have the list sitting around. This commit removes it
and the references from the CONTIBUTING and README files as well.

Author: ahpook

CONTRIBUTING.md

@@ -94,8 +94,6 @@ Please note that all suggestions must adhere to GitHub's [Community Guidelines](
 
 To propose a new topic or collection, please **open a pull request** with your proposed additions. The [API docs](./docs/API.md) and [style guide](./docs/styleguide.md) provide guidance on the information you need to include and how it should be formatted.
 
-This repository includes [a list of the most-used GitHub topics that don't yet have extra context](topics-todo.md). If your pull request adds one of these topics, please update topics-todo.md so that the topic is checked (marked complete).
-
 **Please fill out the pull request template completely.** If you do not fill out the template, your pull request will be closed.
 
 ## Guidelines

README.md

@@ -6,7 +6,7 @@ This repository houses all of the community-curated content for GitHub Topics an
 
 [Collections](https://github.com/collections) help you discover hand-picked repositories, developers, organizations, videos, and articles that share a common theme.
 
-If you want to suggest edits to an existing Topic page or Collection, or curate a new one, read our [contributing guide](CONTRIBUTING.md) to get started. You can also [review a list of popular Topics that need more context](topics-todo.md) to get an idea of where to start.
+If you want to suggest edits to an existing Topic page or Collection, or curate a new one, read our [contributing guide](CONTRIBUTING.md) to get started.
 
 ## Running tests