Fix minor bug in delete_nodes utility due to typo

[sphuber]

Fixes #1564

Added a test that would have caught this bug. For this it was
necessary to set the verbosity to 2, because the bug was only
triggered in the branches of code for high verbosity. Additionally,
the tests created a simple graph of base Node instances, which
means that the logic involving Calculation nodes and CALL links
was not hit. The new test explicitly creates Calculation nodes with
CALL links to test this case

[codecov-io]

Codecov Report

Merging #1565 into develop will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1565      +/-   ##
===========================================
+ Coverage     56.9%   56.96%   +0.05%     
===========================================
  Files          273      273              
  Lines        33862    33862              
===========================================
+ Hits         19270    19288      +18     
+ Misses       14592    14574      -18

Impacted Files	Coverage Δ
aiida/utils/delete_nodes.py	78.04% <ø> (+21.95%)	⬆️

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