Remove unused downloadAll task (#39443)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39443

This task is unused in CI now and we can safely remove it from our build files.

Changelog:
[Internal] [Changed] - Remove unused downloadAll task

Reviewed By: mdvacca, cipolleschi

Differential Revision: D49233339

fbshipit-source-id: 558b44612b71bb1cb220cc21b8339835ae235302