Remove outdated metro type definitions

Summary:
RN itself does not depend on Metro any longer, which is abstracted away into the CLI. I don't think we need those type definitions any longer as we have proper Metro definitions internally. I'm removing them because they keep showing up in biggrep when I look for things.

Changelog: [Internal]

Reviewed By: GijsWeterings

Differential Revision: D21089924

fbshipit-source-id: 2845277af12dae0f0baefaf85adefffb6ef9f2a5