docs: remove broken image from CONTRIBUTING.md (#1396)

Summary:
The image for 'GitHub Actions artifact downloads' in `CONTRIBUTING.md` is broken. The file `static/actions-artifacts.png` referenced by the image link is not in the repo. The broken image should not be displayed until a proper image is available.

Fixes https://github.com/facebook/flipper/issues/1394

### Screenshot
![image](https://user-images.githubusercontent.com/49580849/88140196-e4e45880-cc0e-11ea-9b3e-2a490b6439f9.png)

## Changelog

Broken link from `CONTRIBUTING.md` removed.

Pull Request resolved: https://github.com/facebook/flipper/pull/1396

Test Plan: Visit [`CONTRIBUTING.md`](https://github.com/facebook/flipper/blob/master/CONTRIBUTING.md)

Reviewed By: jknoxville

Differential Revision: D22662057

Pulled By: mweststrate

fbshipit-source-id: 9d2b26196da6078accddcd119bdd005752f46789

CONTRIBUTING.md

@@ -29,8 +29,6 @@ We actively welcome your pull requests.
 
 ### Verifying changes to Flipper
 
-![GitHub Actions artifact downloads](static/actions-artifacts.png)
-
 After opening a pull request or pushing to a branch, the CI will generate
 build artifacts for you for Linux, MacOS and Windows. You can download them
 from the GitHub Actions checks on your Pull Request.