Remove PropTypes from package in favor of newly-supported Flow metadata.json in dash

[mjclawar]

Description

plotly/dash#207 updates to support flow types in v0.21.0

Dash now supports React components that use Flow. To support Flow, component_loader now has the following behavior to create docstrings as determined in discussion in #187: 1. If a Dash component has PropTypes-generated typing, the docstring uses the PropTypes, regardless of whether the component also has Flow types (current behavior). 2. Otherwise if a Dash component has Flow types but not PropTypes, the docstring now uses the objects generated by react-docgen from the Flow types.

We can now remove PropTypes entirely from the package and, where necessary, update Flow to mimic the types.