Remove text builds of documentation (rapidsai#2354)

This PR removes text builds of the documentation, which we do not currently use for anything. Contributes to rapidsai/build-planning#71.

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Ben Frederickson (https://github.com/benfred)
  - James Lamb (https://github.com/jameslamb)

URL: rapidsai#2354

ci/build_docs.sh

@@ -41,10 +41,8 @@ popd
 rapids-logger "Build Python docs"
 pushd docs
 sphinx-build -b dirhtml source _html
-sphinx-build -b text source _text
-mkdir -p "${RAPIDS_DOCS_DIR}/raft/"{html,txt}
+mkdir -p "${RAPIDS_DOCS_DIR}/raft/"html
 mv _html/* "${RAPIDS_DOCS_DIR}/raft/html"
-mv _text/* "${RAPIDS_DOCS_DIR}/raft/txt"
 popd
 
 rapids-upload-docs