Revert changes to testing with new XGBoost release

In #3324 , we had to mark some tests as expected to fail since XGBoost was throwing a FutureWarning. The warning has been addressed in XGBoost, so we're just waiting for the [PR](https://github.com/dmlc/xgboost/pull/7595) merged to be released. This issue is discussed in the #3275 issue.

`evalml/tests/component_tests/test_xgboost_classifier.py` needs to have the `@pytest.mark.xfail` removed from the `test_xgboost_catch_warnings_label_encoder` test when XGBoost releases. EvalML will upgrade automatically when XGBoost releases, which should actually cause these tests to pass (and thus fail because of the xfail mark). Once the xfail is removed, this story is complete.