cleanup(storage): prepare for clang 18

[alevenberg]

#14076

Step #2: /workspace/google/cloud/storage/internal/async/writer_connection_buffered.cc:164:5: error: redundant explicit casting to the same type 'void' as the sub-expression, remove this casting [readability-redundant-casting,-warnings-as-errors]
Step #2:   164 |     (void)StartWriting(std::move(lk));
Step #2:       |     ^~~~~~
Step #2: /workspace/google/cloud/storage/internal/async/writer_connection_buffered.cc:168:8: note: source type originates from the invocation of this method
Step #2:   168 |   void StartWriting(std::unique_lock<std::mutex> lk) {
Step #2:       |   ~~~~ ^
Step #2: 53615 warnings generated.

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.23%. Comparing base (2808049) to head (071340e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14211      +/-   ##
==========================================
- Coverage   93.78%   93.23%   -0.56%     
==========================================
  Files        2293     2206      -87     
  Lines      202921   192151   -10770     
==========================================
- Hits       190313   179155   -11158     
- Misses      12608    12996     +388     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.