Move router-act.ts to e2e-utils and delete outdated duplicate

In #84521, The test helper `router-act.ts` was copied from the
`segment-cache` test suite to the `app-prefetch` test suite. This led to
some confusion as the two copies diverged in #84603. To avoid this
problem, the recently updated file has now been moved to `e2e-utils`,
and the now outdated duplicate has been deleted.

Author: unstubbable

test/e2e/app-dir/app-prefetch/prefetching.stale-times.test.ts

@@ -1,5 +1,5 @@
 import { FileRef, nextTestSetup } from 'e2e-utils'
-import { createRouterAct } from './router-act'
+import { createRouterAct } from 'e2e-utils/router-act'
 import { createTimeController } from './test-utils'
 import { join } from 'path'
 

test/e2e/app-dir/app-prefetch/prefetching.test.ts

@@ -1,8 +1,8 @@
 import { FileRef, nextTestSetup } from 'e2e-utils'
+import { createRouterAct } from 'e2e-utils/router-act'
 import { waitFor, retry } from 'next-test-utils'
 import { NEXT_RSC_UNION_QUERY } from 'next/dist/client/components/app-router-headers'
 import { computeCacheBustingSearchParam } from 'next/dist/shared/lib/router/utils/cache-busting-search-param'
-import { createRouterAct } from './router-act'
 import { createTimeController } from './test-utils'
 import { join } from 'path'