ci: Remove parallelization flag from CI

[crisfeim]

CI was previously failing due to two separate issues:

• SQLite I/O disk error
• Ambiguous NSEntityDescription errors

These issues came from two different levels of parallelization:

• Intra-target parallelization: test methods within the same target running in parallel.
• Inter-target parallelization: test targets running in parallel (introduced with test plans).

✅ Fixes

• The SQLite I/O error was fixed by disabling intra-target parallelization for the EssentialFeedCacheIntegrationTests target, since it mutates shared state (file system).
• The NSEntityDescription ambiguity was fixed by making the Core Data model a static let, ensuring it’s loaded only once in memory (instead of once per test target).

❌ Removing the flag

The -parallel-testing-enabled NO flag globally disables all forms of parallelization, including inter-target, which increases CI times. Since both issues have now been resolved at their root, the flag is no longer necessary.