test(react): Remove duplicated test mock (#20200)

The `getActiveSpan` mock calls `actual.getActiveSpan()` and immediately
assigns to the returned span without guarding
against `undefined`. When the router subscriber fires outside an active
span context, `span` is `undefined` and the property assignment throws a
TypeError.

Additionally, there are two `vi.mock('@sentry/core')` declarations for
the same module; the first (lines 66-73) is dead code since the second
one overrides it.

Closes #20199

Author: s1gr1d

packages/react/test/reactrouter-cross-usage.test.tsx

@@ -63,15 +63,6 @@ vi.mock('@sentry/browser', async requireActual => {
   };
 });
 
-vi.mock('@sentry/core', async requireActual => {
-  return {
-    ...(await requireActual()),
-    getRootSpan: () => {
-      return mockRootSpan;
-    },
-  };
-});
-
 vi.mock('@sentry/core', async requireActual => {
   const actual = (await requireActual()) as any;
   return {
@@ -82,6 +73,8 @@ vi.mock('@sentry/core', async requireActual => {
     getActiveSpan: () => {
       const span = actual.getActiveSpan();
 
+      if (!span) return undefined;
+
       span.updateName = mockNavigationSpan.updateName;
       span.setAttribute = mockNavigationSpan.setAttribute;