Remove tensorflow/keras files and the keras dependency

[priya-sundaram-dev]

Reopens the cleanup from #15418 (my earlier #15419 was auto-closed by the keeper bot for an unmarked checklist — this PR fills it in).

Closes the cleanup discussed in #15418.

These four files aren't algorithms — they're how-to-use scripts that wrap a deep-learning framework, and they were the only thing dragging in the heavy keras/tensorflow stack:

• computer_vision/cnn_classification.py
• dynamic_programming/k_means_clustering_tensorflow.py
• machine_learning/lstm/lstm_prediction.py
• neural_network/input_data.py — a TensorFlow MNIST data loader; git grep confirms nothing under a collected .py path imports it.

I also removed the now-orphaned machine_learning/lstm/ package (only __init__.py + sample_data.csv, which existed solely for lstm_prediction.py).

Cleanups

• Dropped keras>=3.7 from pyproject.toml and ran uv sync --upgrade && uv lock. The lock now resolves 80 packages; removed absl-py, h5py, keras, ml-dtypes, namex, optree (plus minor bumps to fonttools/httpcore2/httpx2 from --upgrade).
• Removed the # keras needs hdf5 on pre-release Python libhdf5-dev install step from both build.yml and sphinx.yml — it only existed for keras.
• Dropped the four now-dead --ignore= entries in build.yml.
• Removed the four DIRECTORY.md entries and the empty Lstm heading.

Verification

• git grep -i -E 'tensorflow|keras' now returns only English dictionary words in strings/anagrams.txt / strings/words.txt (kerasin/kerasine), never the library.
• Both workflow files still parse as valid YAML.

One thing left in place

neural_network/gan.py_tf still import input_data, but its .py_tf extension means it is not collected or executed anywhere, and it's out of scope for this PR. Happy to remove or restore it separately if you'd like.

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Describe your change

• Fix a bug or typo in an existing algorithm? — housekeeping: removes non-algorithm how-to files and the dead keras/tensorflow dependency they dragged in (per Should we remove dependencies keras and tensorflow? #15418).
• Documentation change? — updates DIRECTORY.md.

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. — N/A: this is a coordinated removal of four non-algorithm files + the orphaned lstm/ package, as agreed in Should we remove dependencies keras and tensorflow? #15418.
• All new Python files are placed inside an existing directory. — N/A (no new files).
• 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. — N/A (removal-only).
• All functions have doctests that pass the automated testing. — N/A (removal-only).
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. — N/A (removal-only).