remove unused variables in build_docs script

[jameslamb]

Description

Contributes to rapidsai/build-planning#106

This project doesn't need any of the changes that are being made as part of that issue across most other repos... it's already doing all of its pip and conda installs in CI in single installs (which is strict and safe) 🎉

This small PR just makes its handling of environment variables in ci/build_docs.sh consistent with other RAPIDS repos, in the interest of keeping those scripts looking similar so it's easy to apply changes across all RAPIDS projects. ref: rapidsai/cudf#17013 (comment)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[jameslamb]

I realized that RAPIDS_VERSION and RAPIDS_VERSION_MAJOR_MINOR need to be set in the environment by the build_docs script, because they're used for Doxygen.

cuspatial/cpp/doxygen/Doxyfile

Line 41 in e002dc8

PROJECT_NUMBER = $(RAPIDS_VERSION)

cuspatial/cpp/doxygen/Doxyfile

Line 2174 in e002dc8

TAGFILES = rmm.tag=https://docs.rapids.ai/api/librmm/$(RAPIDS_VERSION_MAJOR_MINOR) "libcudf.tag=https://docs.rapids.ai/api/libcudf/$(RAPIDS_VERSION_MAJOR_MINOR)"

Just pushed a commit restoring them. I think the small diff left in this PR is still worth it, to replace a hard-coded version number with one that's driven by the VERSION file.

[jameslamb]

/merge