Remove unnecessary warning for invalid node

We have an invariant that checks the same case right afterwards.

The warning was originally added in #5884 with a distinct wording.

However it was later changed to the same wording as the invariant in #6008.

I don't see why we would want to have both since they're saying the same thing and with (almost) the same internal stack.

src/renderers/shared/shared/__tests__/ReactStatelessComponent-test.js

@@ -123,30 +123,24 @@ describe('ReactStatelessComponent', () => {
     );
   });
 
-  it('should warn when stateless component returns array', () => {
-    spyOn(console, 'error');
+  it('should throw when stateless component returns array', () => {
     function NotAComponent() {
       return [<div />, <div />];
     }
     expect(function() {
       ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
-    }).toThrow();
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    }).toThrowError(
       'NotAComponent(...): A valid React element (or null) must be returned. ' +
       'You may have returned undefined, an array or some other invalid object.'
     );
   });
 
-  it('should warn when stateless component returns undefined', () => {
-    spyOn(console, 'error');
+  it('should throw when stateless component returns undefined', () => {
     function NotAComponent() {
     }
     expect(function() {
       ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
-    }).toThrow();
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    }).toThrowError(
       'NotAComponent(...): A valid React element (or null) must be returned. ' +
       'You may have returned undefined, an array or some other invalid object.'
     );

src/renderers/shared/stack/reconciler/ReactCompositeComponent.js

@@ -39,26 +39,9 @@ function StatelessComponent(Component) {
 StatelessComponent.prototype.render = function() {
   var Component = ReactInstanceMap.get(this)._currentElement.type;
   var element = Component(this.props, this.context, this.updater);
-  warnIfInvalidElement(Component, element);
   return element;
 };
 
-function warnIfInvalidElement(Component, element) {
-  if (__DEV__) {
-    warning(
-      element === null || element === false || React.isValidElement(element),
-      '%s(...): A valid React element (or null) must be returned. You may have ' +
-      'returned undefined, an array or some other invalid object.',
-      Component.displayName || Component.name || 'Component'
-    );
-    warning(
-      !Component.childContextTypes,
-      '%s(...): childContextTypes cannot be defined on a functional component.',
-      Component.displayName || Component.name || 'Component'
-    );
-  }
-}
-
 function shouldConstruct(Component) {
   return !!(Component.prototype && Component.prototype.isReactComponent);
 }
@@ -205,7 +188,13 @@ var ReactCompositeComponent = {
     // Support functional components
     if (!doConstruct && (inst == null || inst.render == null)) {
       renderedElement = inst;
-      warnIfInvalidElement(Component, renderedElement);
+      if (__DEV__) {
+        warning(
+          !Component.childContextTypes,
+          '%s(...): childContextTypes cannot be defined on a functional component.',
+          Component.displayName || Component.name || 'Component'
+        );
+      }
       invariant(
         inst === null ||
         inst === false ||