Remove unused trace features

[michaelosthege]

What is this PR about?
To prepare refactoring the sample storage backend, this PR simplifies things by removing unused features.

Starting chain numbering at values other than zero is a super rare edge case that isn't tested across all flavors of single/multiprocess sampling, and is likely to break downstream code.
Removing this feature simplifies the logic in sampling.py, thereby making it easier to refactor.

Secondly, I've removed merge_traces and the related merge_reports functions.
This is well covered by idata.extend these days.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• None

Bugfixes / New features

• None

Docs / Maintenance

• Removed support for starting chain numbering at >0.
• Removed non-public helper functions for merging MultiTrace objects.

[codecov]

Codecov Report

Merging #6188 (dcf67e9) into main (faebc60) will decrease coverage by 22.84%.
The diff coverage is 75.00%.

❗ Current head dcf67e9 differs from pull request most recent head 406f173. Consider uploading reports for the commit 406f173 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6188       +/-   ##
===========================================
- Coverage   93.37%   70.52%   -22.85%     
===========================================
  Files         100      100               
  Lines       21896    21843       -53     
===========================================
- Hits        20445    15405     -5040     
- Misses       1451     6438     +4987     

Impacted Files	Coverage Δ
pymc/backends/report.py	90.55% <ø> (-0.50%)	⬇️
pymc/parallel_sampling.py	76.49% <ø> (-9.32%)	⬇️
pymc/tests/backends/test_ndarray.py	100.00% <ø> (ø)
pymc/sampling.py	70.67% <66.66%> (-11.98%)	⬇️
pymc/backends/base.py	85.54% <100.00%> (-0.91%)	⬇️
pymc/tests/smc/test_smc.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/tuning/test_scaling.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/tuning/test_starting.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/step_methods/test_slicer.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/step_methods/hmc/test_nuts.py	0.00% <0.00%> (-100.00%)	⬇️
... and 31 more