[ci] revert clang-18 LD_LIBRARY_PATH workaround (#6559)

.github/workflows/r_package.yml

@@ -316,11 +316,6 @@ jobs:
       - name: Install packages and run tests
         shell: bash
         run: |
-          # patch around library-loading issues in clang18 image
-          # ref: https://github.com/microsoft/LightGBM/issues/6553
-          if [[ "${{ matrix.image }}" == "clang18" ]]; then
-            export LD_LIBRARY_PATH="/usr/lib/llvm-18/lib:${LD_LIBRARY_PATH}"
-          fi
           Rscript -e "install.packages(c('R6', 'data.table', 'jsonlite', 'knitr', 'markdown', 'Matrix', 'RhpcBLASctl', 'testthat'), repos = 'https://cran.rstudio.com', Ncpus = parallel::detectCores())"
           sh build-cran-package.sh
           if [[ "${{ matrix.image }}" =~ "clang" ]]; then