remove conditional_jit from plot_forest label_idx

[OriolAbril]

Description

Related to #2313. This should fix the bug mentioned in that issue.
With the recent improvements to numba it has become more performant yet more restrictive
to support the performance improvements.

plot_forest had a function that was already using forceobj=True, and now due to the
combination of lists, dicts and .append, numba rejects compiling it (probably wasn't doing much
to begin with in previous versions). So I have removed the conditional_jit part.

Checklist

• Does the PR follow official PR format?
• Is the code style correct (follows pylint and black guidelines)?
• Is the fix listed in the Maintenance and fixes
  section of the changelog?

---

📚 Documentation preview 📚: https://arviz--2319.org.readthedocs.build/en/2319/

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 86.79%. Comparing base (82ea6e4) to head (c869d9a).

Files	Patch %	Lines
arviz/data/io_pystan.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2319   +/-   ##
=======================================
  Coverage   86.79%   86.79%           
=======================================
  Files         123      123           
  Lines       12728    12722    -6     
=======================================
- Hits        11047    11042    -5     
+ Misses       1681     1680    -1     

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[drbenvincent]

I can confirm that this fixes the bug I was encountering (#2313)