infra: remove path filter on check_diffs (#15418)

CI should run on https://github.com/langchain-ai/langchain/pull/15412

But github only checks first 300 files:
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#git-diff-comparisons

> Diffs are limited to 300 files. If there are files changed that aren't
matched in the first 300 files returned by the filter, the workflow will
not run. You may need to create more specific filters so that the
workflow will run automatically.