dillon1000/react

Remove injectMixin (#6831)

Summary:
The only callsite left was removed here: https://github.com/facebook/react/commit/e8af100849932387643977587ccd7a4c1a981470#commitcomment-17570210 but the code to handle it remained.

Test Plan:
tbgs and ibgs, make sure there's no callsites.

Reviewers: @jimfb, @spicyj, @sebmarkbage