fixed task handling when cancelled

[mtitov]

when task is cancelled it shouldn't be handled within the Popen._check_running method

(*) revert back this change 2e6f186

[andre-merzky]

my bad, thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 43.28%. Comparing base (9aa29b1) to head (c52cadb).
Report is 4 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/radical/pilot/agent/executing/popen.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #3304      +/-   ##
==========================================
+ Coverage   43.27%   43.28%   +0.01%     
==========================================
  Files          97       97              
  Lines       11459    11460       +1     
==========================================
+ Hits         4959     4961       +2     
+ Misses       6500     6499       -1     

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