[Internal API only] Delete non-awaited form of act

**This commit only affects the internal version of `act` that we use in
this repo. The public `act` API is unaffected, for now.**

We should always await the result of an `act` call so that any work
queued in a microtask has a chance to flush. Neglecting to do this can
cause us to miss bugs when testing React behavior.

I codemodded all the existing `act` callers in previous PRs.

packages/jest-react/src/internalAct.js

@@ -69,53 +69,42 @@ export function act<T>(scope: () => Thenable<T> | T): Thenable<T> {
   // our test suite, we should be able to.
   try {
     const result = scope();
+    const thenableResult: Thenable<T> = (result: any);
+
     if (
-      typeof result === 'object' &&
-      result !== null &&
-      // $FlowFixMe[method-unbinding]
-      typeof result.then === 'function'
+      typeof thenableResult !== 'object' ||
+      thenableResult === null ||
+      typeof thenableResult.then !== 'function'
     ) {
-      const thenableResult: Thenable<T> = (result: any);
-      return {
-        then(resolve: T => mixed, reject: mixed => mixed) {
-          thenableResult.then(
-            returnValue => {
-              flushActWork(
-                () => {
-                  unwind();
-                  resolve(returnValue);
-                },
-                error => {
-                  unwind();
-                  reject(error);
-                },
-              );
-            },
-            error => {
-              unwind();
-              reject(error);
-            },
-          );
-        },
-      };
-    } else {
-      const returnValue: T = (result: any);
-      try {
-        // TODO: Let's not support non-async scopes at all in our tests. Need to
-        // migrate existing tests.
-        let didFlushWork;
-        do {
-          didFlushWork = Scheduler.unstable_flushAllWithoutAsserting();
-        } while (didFlushWork);
-        return {
-          then(resolve: T => mixed, reject: mixed => mixed) {
-            resolve(returnValue);
-          },
-        };
-      } finally {
-        unwind();
-      }
+      throw new Error(
+        'The internal version of `act` used in the React repo must be passed ' +
+          "an async function, even if doesn't await anything. This is a " +
+          'temporary limitation that will soon be fixed.',
+      );
     }
+
+    return {
+      then(resolve: T => mixed, reject: mixed => mixed) {
+        thenableResult.then(
+          returnValue => {
+            flushActWork(
+              () => {
+                unwind();
+                resolve(returnValue);
+              },
+              error => {
+                unwind();
+                reject(error);
+              },
+            );
+          },
+          error => {
+            unwind();
+            reject(error);
+          },
+        );
+      },
+    };
   } catch (error) {
     unwind();
     throw error;