Combine file checks with build checks back into a single workflow file (#1058)

Previously, I tried to make ci-file-checks.yaml be efficient and skip
running some checks when they were not relevant to the files changed in
a PR. However, the maintenance has been higher than hoped, and other
team members didn't like the arrangement. Moreover, the overall CI
workflow execution time for TFQ is bounded by the build checks, so
skipping some file checks did not reduce overall CI time. While it _is_
obviously best to save CPU cycles (= electricity = energy) as much as
possible by not running unnecessary programs, the tradeoffs still argue
against keeping the current ci-file-checks.yaml approach.

So, this PR:

- simplifies the file checks into a single job
- integrates that file check job into ci-build-checks.yaml
- renames the combined result to `ci.yaml`
- removes ci-file-checks.yaml
- removes the use of GitHub problem-matchers, to simplify things further

Author: mhucka