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%)