Update PatchOrdering.kt

[GeorgCantor]

Description

1. Early Return for DELETE Type
   We immediately return an empty set if the patch type is DELETE, eliminating unnecessary nesting.
2. Use of flatMap
   Instead of nested loops, we use flatMap to streamline the process of mapping IDs to their corresponding references. This reduces complexity and improves readability.
3. Avoiding Redundant Calls
   The use of ?: emptyList() ensures that we handle cases where an ID might not be present in the map without additional checks.

Type
Code health

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[jingtang10]

You need to run spotless before this can be merged. See https://google.github.io/android-fhir/contrib/style/.