Removes unused environment variables from Morpheus build docs

[yczhang-nv]

Description

Removes the following environment variables (that are never used during Morpheus build process) from documentation: PYTHON_VER RAPIDS_VER TRITONCLIENT_VERSION CUDA_VER Closes #1781

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[yczhang-nv]

/merge