Removed unnecessary warnings as described in #941

[zuari1993]

Description

What was done:

• Fixes #941 (Disable unnecessary warning in EnsembleVoteClassifier)
• Added it to changlog

• 980 'Enhancement: Zhangs metric' was placed in the wrong section in the change-log (was put under new features section of version v0.20.0, I moved it in to the right section (under new features section of v0.21.0)

Pull Request Checklist

• [x] Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• [x] Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• [x] Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• [x] 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)
• [x] Checked for style issues by running flake8 ./mlxtend

[codecov[bot]]

Codecov Report

Base: 77.45% // Head: 77.46% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (66d5484) compared to base (55359c7). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #994 +/- ## ========================================== + Coverage 77.45% 77.46% +0.01% ========================================== Files 198 198 Lines 11171 11171 Branches 1406 1406 ========================================== + Hits 8652 8654 +2 + Misses 2305 2304 -1 + Partials 214 213 -1 ``` | [Impacted Files](https://codecov.io/gh/rasbt/mlxtend/pull/994?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [mlxtend/classifier/ensemble\_vote.py](https://codecov.io/gh/rasbt/mlxtend/pull/994/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9jbGFzc2lmaWVyL2Vuc2VtYmxlX3ZvdGUucHk=) | `71.60% <100.00%> (ø)` | | | [mlxtend/evaluate/counterfactual.py](https://codecov.io/gh/rasbt/mlxtend/pull/994/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9jb3VudGVyZmFjdHVhbC5weQ==) | `100.00% <0.00%> (+6.89%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka)

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