Remove default branch logic

[jtpio]

Follow-up to #192

This logic was picking up the master branch of the releaser repo.

With this change we remove the explicit branch name and let the forwardport_changelog choose the default branch if needed.

This seems to work with the Publish Release workflow. Not sure about the Full Release.

[codecov-commenter]

Codecov Report

Merging #195 (aebdf19) into master (e6f503c) will increase coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   84.77%   84.85%   +0.07%     
==========================================
  Files          19       19              
  Lines        2273     2271       -2     
  Branches      280      280              
==========================================
  Hits         1927     1927              
+ Misses        250      248       -2     
  Partials       96       96              

Impacted Files	Coverage Δ
jupyter_releaser/actions/publish_release.py	0.00% <0.00%> (ø)

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[blink1073]

Thanks!

[jtpio]

Gonna make a new 0.9.5 release with this to try it out on a repo right after.

[jtpio]

This Full Release run went all through with the latest releaser and a repo with main as the default branch: https://github.com/jtpio/jupyter_releaser/runs/3976982945?check_suite_focus=true