test: don't run tests with specific XML parser

We have `test/test_parser.rb` that runs tests for all available
parsers.

Author: kou

.github/workflows/test.yml

@@ -38,9 +38,4 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true # 'bundle install' and cache gems
-      - name: Test with REXML
-        run: bundle exec rake
-      - name: Test with libxml
-        env:
-          XMLRPC_PARSER: libxml
-        run: bundle exec rake
+      - run: bundle exec rake