Remove a workaround about rmarkdown on R <3.6 (#4069)

Author: yutannihilation

.github/workflows/R-CMD-check.yaml

@@ -77,11 +77,6 @@ jobs:
           sysreqs=$(Rscript -e "cat(sysreqs::sysreq_commands('DESCRIPTION'))")
           sudo -s eval "$sysreqs"
 
-      # Remove this when the new rmarkdown is released on CRAN
-      - name: "Workaround for rstudio/rmarkdown#1831"
-        if: runner.os == 'Linux' && matrix.config.r < '3.6'
-        run: Rscript -e "remotes::install_github('rstudio/rmarkdown')"
-
       - name: Install system dependencies on macOS
         if: runner.os == 'macOS'
         run: |