[flow] Convert all remaining Shape to $Partial

Summary:
Alongside my attempts to replace $Shape with Partial in the preceding diffs, I'm also looking to replace our custom type Shape.

This diff handles all remaining cases, and deletes the lib/types/core.js file, whose only contents was the Shape type.

Depends on D10086

Test Plan: Flow

Reviewers: michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10087