Prop types validation should not happen twice

[sophiebits]

Historically, we validated prop types at component mount time, but we switched to validating them upon element creation because it gives earlier errors and better stack traces. We don't want to ever show a message with "Failed Composite propType:" any more, so we can remove the code that generates it. In addition, we should remove this check from fbjs that will be unnecessary after the code cleanup:

https://github.com/facebook/fbjs/blob/71392a58a2cbac228ffd535e9ebd7072a20acb73/src/__forks__/warning.js#L34

This should mildly improve performance in DEV mode.