Update docs build script (#8284)

This PR removes a variable that is no longer necessary for docs builds after the calver transition.

Authors:
  - AJ Schmidt (https://github.com/ajschmidt8)

Approvers:
  - Dillon Cullinan (https://github.com/dillon-cullinan)

URL: #8284

ci/docs/build.sh

@@ -15,7 +15,6 @@ export PATH=/conda/bin:/usr/local/cuda/bin:$PATH
 export HOME=$WORKSPACE
 export PROJECT_WORKSPACE=/rapids/cudf
 export LIBCUDF_KERNEL_CACHE_PATH="$HOME/.jitify-cache"
-export NIGHTLY_VERSION=$(echo $BRANCH_VERSION | awk -F. '{print $2}')
 export PROJECTS=(cudf libcudf)
 
 gpuci_logger "Check environment..."