Remove Babylon plugins for features already merged to the ECMAScript spec (#8448)

These are now enabled by default:
- objectRestSpread (2018)
- asyncGenerators (2018)
- optionalCatchBInding (2019)
- jsonStrings (2019)

TODO (after this commit):
- [ ] Deprecate the `@babel/plugin-syntax-*` packages.
- [ ] Deprecate the `@babel/plugin-proposal-*` packages.
- [ ] Create the `@babel/plugin-transform-*` packages.

scripts/tests/flow/run_babel_parser_flow_tests.js

@@ -109,12 +109,10 @@ function update_whitelist(summary) {
 
 const options = {
   plugins: [
-    "asyncGenerators",
     "dynamicImport",
     ["flow", { all: true }],
     "flowComments",
     "jsx",
-    "objectRestSpread",
     "classPrivateProperties",
   ],
   sourceType: "module",