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.

packages/babel-parser/test/fixtures/es2018/async-generators/class-method/input.js

@@ -0,0 +1,7 @@
+class Query {
+    async *queryAll(ids) {
+        for (const id of ids) {
+            yield await this.query(id);
+        }
+    }
+}