remove support for environment files

[ras44]

Describe the bug Maintaining environment files in envs/ and their builds via .github/workflows/test-conda-venv-install.py

https://github.com/uber/causalml/blob/a0315660d9b14f5d943aa688d8242eb621d2ba76/.github/workflows/test-conda-venv-install.yml#L1-L68

is cumbersome and not easily addressed across platforms due to cython and other platform-specific build dependencies.

To Reproduce See failing actions

Proposal

• Remove support for env files
  • Remove envs/
  • Remove .github/workflows/test-conda-venv-install.py
  • Update docs/installation.rst and remove envs installation steps
  • Nudge user to export env to maintain stability
  • Recommend install via conda/pip/build-from-source