Remove isOSS guards from Fantom renderer tests

Summary:
Remove the isOSS conditionals that were skipping Fantom renderer tests in OSS environments. Now that the React Native renderers have been upgraded and the code is compatible in OSS, these tests can run in both internal and OSS environments.

Changes:
- ResponderEventTarget-itest.js: Removed isOSS check that skipped the entire Responder System test suite
- EventTargetDispatching-itest.js: Removed isOSS check that skipped the entire EventTarget-based Event Dispatching test suite
- EventDispatching-benchmark-itest.js: Removed if (isOSS) block that provided a dummy test instead of the benchmark suite
- EventDispatching-itest.js: Removed if (!isOSS) condition guarding three tests for native event timeStamp and global event dispatching

Changelog: [Internal]

Differential Revision: D107675643

Author: rubennorte