fix regression introduced in Spotify bugfix

[rtertiaer]

This PR fixes a bug introduced during #565 ; we don't use the outputs of communicate, and kill doesn't even output. The tests failed on this; it's not clear why Github let me merge anyhow, when our branch protections require passing tests for everybody, including administrators 🥴

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2ca1da8) 51.53% compared to head (ce0dd7f) 51.60%.
Report is 1 commits behind head on develop.

Files	Patch %	Lines
amplipi/streams.py	60.00%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #566      +/-   ##
===========================================
+ Coverage    51.53%   51.60%   +0.07%     
===========================================
  Files           24       24              
  Lines         5676     5676              
===========================================
+ Hits          2925     2929       +4     
+ Misses        2751     2747       -4     

Flag	Coverage Δ
unittests	51.60% <60.00%> (+0.07%)	⬆️

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[linknum23]

I think this needs further explanation. Why was it failing?

[linknum23]

TLDR: It worked fine but the linter caught some unused and non-existent return values which were reported as errors.

[rtertiaer]

this is deployed & waiting on others to test on the appliance in-house; then stamping out 0.3.0 🎊