Remove xgboost demos and dependency (#15219)

Drop machine_learning/xgboost_classifier.py and
machine_learning/xgboost_regressor.py. Both were thin "how-to-use"
wrappers around sklearn's XGBClassifier/XGBRegressor rather than
from-scratch implementations, and the classifier's only doctest was
already disabled (# THIS TEST IS BROKEN!!), so it was never exercised
in CI.

xgboost is one of the heaviest compiled dependencies in the tree (large
wheel, needs OpenMP/libgomp at runtime, no free-threaded wheel yet), and
gradient boosting is already implemented from scratch in
machine_learning/gradient_boosting_classifier.py and
gradient_boosting_regressor.py, so no algorithm coverage is lost.

Removes the xgboost dependency from pyproject.toml, its (and its
xgboost-only transitive dep nvidia-nccl-cu13) entries from uv.lock, and
the two DIRECTORY.md links.

Refs #15081

Author: priya-sundaram-dev

DIRECTORY.md

@@ -696,8 +696,6 @@
   * [Support Vector Machines](machine_learning/support_vector_machines.py)
   * [T Stochastic Neighbour Embedding](machine_learning/t_stochastic_neighbour_embedding.py)
   * [Word Frequency Functions](machine_learning/word_frequency_functions.py)
-  * [Xgboost Classifier](machine_learning/xgboost_classifier.py)
-  * [Xgboost Regressor](machine_learning/xgboost_regressor.py)
 
 ## [Maths](maths)
   * [Abs](maths/abs.py)

machine_learning/xgboost_classifier.py

@@ -27,7 +27,6 @@ dependencies = [
   "statsmodels>=0.14.4",
   "sympy>=1.13.3",
   "typing-extensions>=4.12.2",
-  "xgboost>=2.1.3",
 ]
 
 [dependency-groups]