fix: remove unused import of maxtext_google from train.py

[kryvokhyzha]

Description

Remove import from maxtext import maxtext_google that was introduced in commit 4ef205efe3c4f31a700ff7621e7f1502575bd72c (merge of PR #3189).

• Problem: Running python3 -m maxtext.trainers.pre_train.train fails immediately with ImportError: cannot import name 'maxtext_google' from 'maxtext'. No maxtext_google module exists in the open-source repo - it is a Google-internal side-effect import.
• Fix: Remove the two lines (# pylint: disable-next=unused-import and from maxtext import maxtext_google). The import was unused (pylint disable confirms this) and served only as an internal instrumentation hook.

Error in commit 4ef205efe3c4f31a700ff7621e7f1502575bd72c:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/deps/src/maxtext/trainers/pre_train/train.py", line 46, in <module>
    from maxtext import maxtext_google
ImportError: cannot import name 'maxtext_google' from 'maxtext' (unknown location)

Tests

# Verify the import error is gone
python3 -c "import maxtext.trainers.pre_train.train"

Checklist

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[bvandermoon]

Thank you @kryvokhyzha for sending out this PR! We also have a fix going here: #3202

[kryvokhyzha]

Thank you @kryvokhyzha for sending out this PR! We also have a fix going here: #3202

Thanks, I'll close this PR