CLN: Remove convert_objects

[mroeschke]

• xref DEPR: Clean up list of deprecations from prior versions #6581
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #26612 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26612      +/-   ##
==========================================
- Coverage   91.85%   91.83%   -0.03%     
==========================================
  Files         174      174              
  Lines       50722    50718       -4     
==========================================
- Hits        46593    46579      -14     
- Misses       4129     4139      +10

Flag	Coverage Δ
#multiple	90.38% <ø> (-0.02%)	⬇️
#single	41.78% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/generic.py	93.61% <ø> (-0.02%)	⬇️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/dtypes/cast.py	90.54% <0%> (-1%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

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[jreback]

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=12188

looks like a doc-string test still has this, otherwise lgtm.

[jreback]

thanks @mroeschke