Cleanup akka-http client usages.

[markusthoemmes]

Was searching for a bug and cleaned up a little meanwhile... No change in behavior intended.

[codecov-io]

Codecov Report

Merging #3870 into master will decrease coverage by 0.01%.
The diff coverage is 85%.

@@            Coverage Diff             @@
##           master    #3870      +/-   ##
==========================================
- Coverage   74.46%   74.45%   -0.02%     
==========================================
  Files         138      137       -1     
  Lines        6478     6471       -7     
  Branches      409      408       -1     
==========================================
- Hits         4824     4818       -6     
+ Misses       1654     1653       -1

Impacted Files	Coverage Δ
.../scala/whisk/core/database/CouchDbRestClient.scala	87.23% <100%> (ø)	⬆️
.../src/main/scala/whisk/http/PoolingRestClient.scala	89.28% <80%> (-1.9%)	⬇️

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[dubee]

PG2 3365 ⏳

[rabbah]

LGTM.

[dubee]

PG2 3365 🔵