remove useless shebang lines

[fabianegli]

The shebang lines are useful for python files which are run as scripts in the console. Many of the files in a package are not. This PR removes the shebang line from the files that do not serve as scripts.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated

[codecov]

Codecov Report

Merging #1973 (00314a7) into dev (6bf8f30) will decrease coverage by 0.97%.
The diff coverage is n/a.

❗ Current head 00314a7 differs from pull request most recent head 5e9fcae. Consider uploading reports for the commit 5e9fcae to get more accurate results

@@            Coverage Diff             @@
##              dev    #1973      +/-   ##
==========================================
- Coverage   62.01%   61.03%   -0.98%     
==========================================
  Files          42       37       -5     
  Lines        4862     4666     -196     
==========================================
- Hits         3015     2848     -167     
+ Misses       1847     1818      -29     

Impacted Files	Coverage Δ
nf_core/bump_version.py	89.47% <ø> (-0.19%)	⬇️
nf_core/download.py	52.72% <ø> (-0.12%)	⬇️
nf_core/launch.py	63.58% <ø> (-0.10%)	⬇️
nf_core/licences.py	22.85% <ø> (-1.09%)	⬇️
nf_core/lint/modules_structure.py	88.88% <ø> (ø)
nf_core/lint/multiqc_config.py	66.66% <ø> (ø)
nf_core/lint/pipeline_name_conventions.py	81.81% <ø> (ø)
nf_core/lint/pipeline_todos.py	94.11% <ø> (ø)
nf_core/lint/readme.py	75.75% <ø> (ø)
nf_core/lint/schema_description.py	67.85% <ø> (ø)
... and 37 more

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