COVERAGE: Cleanup files.PublishedStorage after dropping a publish

[neolynx]

After dropping files.PublishedStorage there were some leftovers - empty directory tree. If that directory tree is empty, it is now removed.

Fixes: #198

Fixes #

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.

Project coverage is 74.91%. Comparing base (0e6f9c3) to head (c8e2198).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
deb/publish.go	63.63%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1382      +/-   ##
==========================================
+ Coverage   74.84%   74.91%   +0.06%     
==========================================
  Files         151      151              
  Lines       17161    17180      +19     
==========================================
+ Hits        12845    12871      +26     
+ Misses       3292     3284       -8     
- Partials     1024     1025       +1     

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