Remove deprecated STAC_IO class

[duckontheweb]

Related Issue(s):

• RFC: Remove deprecated STAC_IO class #431

Description:

Removes the STAC_IO class that was deprecated in v1.0.0-beta.1

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 #490 (c0a2c9b) into main (f9573f8) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
+ Coverage   91.66%   91.77%   +0.10%     
==========================================
  Files          40       40              
  Lines        5245     5192      -53     
==========================================
- Hits         4808     4765      -43     
+ Misses        437      427      -10     

Impacted Files	Coverage Δ
pystac/stac_io.py	83.47% <ø> (+0.71%)	⬆️

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