chore: Remove LangChain Python 3.8 unit tests as a required check

PiperOrigin-RevId: 704833710

Author: yeesian

.github/sync-repo-settings.yaml

@@ -22,7 +22,6 @@ branchProtectionRules:
  - 'Presubmit - Unit Tests Python 3.11'
  - 'Presubmit - Unit Tests Python 3.12'
  - 'Presubmit - Unit Tests Ray 2.9.3'
- - 'Presubmit - Unit Tests LangChain (Python 3.8)'
  - 'Presubmit - Unit Tests LangChain (Python 3.9)'
  - 'Presubmit - Unit Tests LangChain (Python 3.10)'
  - 'Presubmit - Unit Tests LangChain (Python 3.11)'