Brev: Remove unused Docker-in-Docker to reduce image size.

[brycelelbach]

Summary

• Remove Docker-in-Docker installation from the accelerated-python, cuda-cpp, and nvmath-python Brev dockerfiles.
• Docker was unused inside these containers and removing it reduces image size.

$ brev/test-docker-image-sizes.bash compare --owner nvidia
 --tutorial accelerated-python --tag pull-request-156-latest --baseline main-latest
========================================
Image: ghcr.io/nvidia/accelerated-python-tutorial
========================================
  Current    (pull-request-156-latest)                12.53 GB
  Baseline   (main-latest)                            12.66 GB
  ────────────────────────────────────────────────────
  Change:  -130.09 MB (-1.00%)

[github-actions]

❌ Link Check Failed

Broken links were detected in this PR.

Please check the workflow run logs for details on which links are broken.

Common fixes:

1. Typo in URL - Check for spelling mistakes in the link
2. Outdated link - The page may have moved or been deleted
3. Relative path issue - Ensure relative links use the correct path
4. External site down - If the external site is temporarily down, you can add it to brev/.lycheeignore

To test links locally:

./brev/test-links.bash .

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