chore(instance): extract server delete command from custom_server.go

[Codelax]

Little refactor to make #4149 readable. I only did a copy, there is no logical changes.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.13812% with 45 lines in your changes missing coverage. Please review.

Project coverage is 70.88%. Comparing base (5bf53e0) to head (13ed6b0).
Report is 399 commits behind head on master.

Files with missing lines	Patch %	Lines
...nal/namespaces/instance/v1/custom_server_delete.go	75.13%	37 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4366      +/-   ##
==========================================
- Coverage   75.45%   70.88%   -4.57%     
==========================================
  Files         202      296      +94     
  Lines       44323    65533   +21210     
==========================================
+ Hits        33444    46454   +13010     
- Misses       9653    17448    +7795     
- Partials     1226     1631     +405     

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