[DOC] RDoc is not in ./lib now

It is already a bundled gem, and only the non-built-in parts remain in
pathname.rb in ./lib and cannot work in old versions.

Author: nobu

.github/workflows/check_misc.yml

@@ -64,7 +64,7 @@ jobs:
       - name: Generate docs
         id: docs
         run: |
-          ruby -W0 --disable-gems -I./lib tool/rdoc-srcdir -q --op html .
+          ruby -W0 --disable-gems tool/rdoc-srcdir -q --op html .
           echo htmlout=ruby-html-${GITHUB_SHA:0:10} >> $GITHUB_OUTPUT
         # Generate only when document commit/PR
         if: >-