Remove @babel/plugin-transform-object-assign from preprocessor

Summary:
React Native has an *implicit* dev dependency on this transform via `metro-react-native-babel-transformer`. The transform replaces `Object.assign` with `babelHelpers.extend`, but `Object.assign` has been available natively since node 4.

We intend remove it from metro (https://github.com/facebook/metro/pull/745) as it's no longer needed by any supported runtime - removing RN's small dependency in advance so RN's tests won't break when we do.

Changelog:
[Internal][Changed] - Remove `babel/plugin-transform-object-assign` from jest preprocessor

Reviewed By: motiz88

Differential Revision: D34110208

fbshipit-source-id: 064f8241461fb338de1cd8b53077e8660301aa77

jest/preprocessor.js

@@ -92,7 +92,6 @@ module.exports = {
           {loose: true}, // dont 'a'.concat('b'), just use 'a'+'b'
         ],
         [require('@babel/plugin-transform-exponentiation-operator')],
-        [require('@babel/plugin-transform-object-assign')],
         [require('@babel/plugin-transform-for-of'), {loose: true}],
         [require('@babel/plugin-transform-react-display-name')],
         [require('@babel/plugin-transform-react-jsx-source')],