Remove doctest in xgboost_regressor.py main function

[tianyizheng02]

Describe your change:

Contributes to #9718 by removing the doctest from the main function in machine_learning/xgboost_regressor.py. The main function is just driver code, and there's no need to run the driver in builds, especially when it's the bottleneck in this file's doctest runtime.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[tianyizheng02]

xgboost_regressor.py's main function is no longer one of the slowest durations:

============================= slowest 10 durations =============================
21.33s call     neural_network/simple_neural_network.py::neural_network.simple_neural_network.forward_propagation
1.53s call     web_programming/get_imdbtop.py::web_programming.get_imdbtop.get_imdb_top_movies
1.04s call     dynamic_programming/integer_partition.py::dynamic_programming.integer_partition.partition
1.01s call     other/dijkstra_bankers_algorithm.py::other.dijkstra_bankers_algorithm.BankersAlgorithm.main
0.96s call     web_programming/fetch_anime_and_play.py::web_programming.fetch_anime_and_play.get_anime_episode
0.91s call     graphs/bidirectional_a_star.py::graphs.bidirectional_a_star.AStar
0.82s call     web_programming/fetch_anime_and_play.py::web_programming.fetch_anime_and_play.search_anime_episode_list
0.79s call     web_programming/fetch_anime_and_play.py::web_programming.fetch_anime_and_play.search_scraper
0.55s call     matrix/count_negative_numbers_in_sorted_matrix.py::matrix.count_negative_numbers_in_sorted_matrix.generate_large_matrix
0.46s call     matrix/count_negative_numbers_in_sorted_matrix.py::matrix.count_negative_numbers_in_sorted_matrix.count_negatives_brute_force
================= 1615 passed, 3 warnings in 69.85s (0:01:09) ==================

[cclauss]

I would not have a problem with this test if it was fast but it is not.