Remove Boxcar due to it no longer being available upstream

[caronc]

Description:

Related issue (if applicable): #1219

Boxcar shutdown in Feb 2019 due to incompatibilties with iOS 10 they no longer wished to support; last X (formerly Twitter) post:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.36%. Comparing base (19c0d8a) to head (299deb1).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1222      +/-   ##
==========================================
- Coverage   99.37%   99.36%   -0.01%     
==========================================
  Files         148      147       -1     
  Lines       20675    20554     -121     
  Branches     3679     3663      -16     
==========================================
- Hits        20545    20424     -121     
  Misses        121      121              
  Partials        9        9              

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