postrm: remove aptly-api user and home directory on purge

[neolynx]

Description of the Change

according to debian policy, purge should leave no trace.

this change removes the user and aptly data on apt-get purge.

See: https://piuparts.debian.org/sid-strict/fail/aptly-api_1.6.0+ds1-3.log

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.03%. Comparing base (666b5c9) to head (1a0bffd).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1425   +/-   ##
=======================================
  Coverage   75.03%   75.03%           
=======================================
  Files         158      158           
  Lines       18356    18356           
=======================================
  Hits        13773    13773           
  Misses       3445     3445           
  Partials     1138     1138           

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