Delete empty descriptors after relocation

[kokorin]

Closes #55

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.74%. Comparing base (6a0d5ec) to head (9aa7809).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dbt_pumpkin/plan.py	96.15%	1 Missing ⚠️
dbt_pumpkin/planner.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   92.70%   92.74%   +0.04%     
==========================================
  Files          10       10              
  Lines         781      813      +32     
==========================================
+ Hits          724      754      +30     
- Misses         57       59       +2     

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