Remove unused variables in aws_host_resolver

[graebm]

Once upon a time, there was a feature call host listeners. This feature wasn't used, so it was later removed. I found some lingering variables. Deleting.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.56%. Comparing base (bca5740) to head (9f113ae).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #719      +/-   ##
==========================================
- Coverage   78.58%   78.56%   -0.02%     
==========================================
  Files          30       30              
  Lines        6388     6383       -5     
==========================================
- Hits         5020     5015       -5     
  Misses       1368     1368              

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