Flow: Remove inferrable types (#3096)

Author: IvanGoncharov

src/__testUtils__/kitchenSinkSDL.js

@@ -1,4 +1,4 @@
-export const kitchenSinkSDL: string = `
+export const kitchenSinkSDL = `
 """This is a description of the schema as a whole."""
 schema {
   query: QueryType

src/jsutils/__tests__/toObjMap-test.js

@@ -5,7 +5,7 @@ import type { ObjMapLike } from '../ObjMap';
 import { toObjMap } from '../toObjMap';
 
 // Workaround to make both ESLint and Flow happy
-const __proto__: string = '__proto__';
+const __proto__ = '__proto__';
 
 describe('toObjMap', () => {
   it('convert empty object to ObjMap', () => {