chore: Remove unused __str__ methods

[l0b0]

They were only accessed by tests.

Related Issue(s): #505

Description:

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #514 (8b969ac) into main (83a6598) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #514      +/-   ##
==========================================
+ Coverage   94.19%   94.24%   +0.05%     
==========================================
  Files          71       71              
  Lines       10302    10272      -30     
  Branches     1072     1072              
==========================================
- Hits         9704     9681      -23     
+ Misses        430      423       -7     
  Partials      168      168              

Impacted Files	Coverage Δ
pystac/catalog.py	91.60% <ø> (-0.05%)	⬇️
pystac/collection.py	93.43% <ø> (+0.33%)	⬆️
pystac/extensions/file.py	92.18% <ø> (-0.13%)	⬇️
pystac/extensions/label.py	93.69% <ø> (+0.13%)	⬆️
pystac/extensions/raster.py	89.37% <ø> (+0.56%)	⬆️
pystac/extensions/scientific.py	95.00% <ø> (+0.55%)	⬆️
pystac/extensions/version.py	96.22% <ø> (-0.07%)	⬇️
pystac/media_type.py	100.00% <ø> (+5.88%)	⬆️
pystac/rel_type.py	100.00% <ø> (ø)
pystac/stac_object.py	90.86% <ø> (-0.10%)	⬇️
... and 4 more

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