dillon1000/react

Remove feature flag around 'getDerivedStateFromProps' bug fix (#13022)

**what is the change?:**
Basically undoes https://github.com/facebook/react/commit/4b2e65d32e3adec7d4ef5ddd5fcb35e617e0cde6#diff-904ceabd8a1e9a07ab1d876d843d62e1

**why make this change?:**
We rolled out this fix internally and in open source weeks ago, and now
we're cleaning up.

**test plan:**
Ran tests and lint, and really we have been testing this because the
flag is open internally as of last week or so.

**issue:**
Internal task T29948812 has some info.