Remove enabling of DeprecationWarning for anyone who imports the library

[gideonred]

Description

At my current company someone imported this library.

Due to the library enabling all DeprecationWarnings on for all of code (including outside of this library), it causes a large increase in logs. Which impacts performance and cost of Observability tooling.

This PR disables that.

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• Ran PYTHONPATH='.' pytest ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend