Remove faulty mypy annotation

[jpolchlo]

Description:
There's a lint error generated with newer mypy versions. This fix removes the erring annotation.

PR Checklist:

• pre-commit hooks pass locally
• 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]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c871b36) 92.03% compared to head (39fc01e) 92.03%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1166   +/-   ##
=======================================
  Coverage   92.03%   92.03%           
=======================================
  Files          49       49           
  Lines        6658     6658           
  Branches      984      984           
=======================================
  Hits         6128     6128           
  Misses        356      356           
  Partials      174      174           

Impacted Files	Coverage Δ
pystac/validation/__init__.py	79.36% <ø> (ø)

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