cleanup(pubsub): no AckResult alias

[dbolduc]

Part of the work for #4508

Remove a less-than-helpful alias.

Not a break, because this hasn't been released.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.05%. Comparing base (0fc24aa) to head (7d3972f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4981      +/-   ##
==========================================
- Coverage   93.06%   93.05%   -0.02%     
==========================================
  Files         221      221              
  Lines        8611     8611              
==========================================
- Hits         8014     8013       -1     
- Misses        597      598       +1     

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