Remove redundant test code in simtel event source test code

[maxnoe]

The exact same code is further down in two individual tests.

[codecov]

Codecov Report

Merging #1747 (de8b96e) into master (9fb2901) will decrease coverage by 0.12%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master    #1747      +/-   ##
==========================================
- Coverage   91.62%   91.49%   -0.13%     
==========================================
  Files         185      185              
  Lines       14382    14380       -2     
==========================================
- Hits        13177    13157      -20     
- Misses       1205     1223      +18     

Impacted Files	Coverage Δ
ctapipe/io/tests/test_simteleventsource.py	100.00% <ø> (ø)
ctapipe/tools/tests/test_tools.py	97.16% <ø> (ø)
ctapipe/conftest.py	95.41% <94.11%> (-1.62%)	⬇️
ctapipe/io/tests/test_dl1eventsource.py	98.73% <100.00%> (+3.73%)	⬆️
ctapipe/tools/tests/test_merge.py	100.00% <100.00%> (ø)
ctapipe/tools/camdemo.py	55.68% <0.00%> (-20.46%)	⬇️
ctapipe/io/dl1eventsource.py	90.60% <0.00%> (-0.56%)	⬇️

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