Do not use vendored libraries

--- a/vendor/CMakeLists.txt
+++ b/vendor/CMakeLists.txt
@@ -18,8 +18,6 @@
 if(GRN_WITH_BUNDLED_ONIGMO)
   add_subdirectory(onigmo)
 endif()
-add_subdirectory(mruby)
-add_subdirectory(mecab)
 if(GRN_WITH_MRUBY)
   add_subdirectory(groonga-log)
 endif()