Ensure script/plugin doesn't show error message when svn externals are not there

railties/lib/commands/plugin.rb

@@ -169,10 +169,13 @@ def uninstall
     else
       puts "Plugin doesn't exist: #{path}"
     end
-    # clean up svn:externals
-    externals = rails_env.externals
-    externals.reject!{|n,u| name == n or name == u}
-    rails_env.externals = externals
+
+    if rails_env.use_externals?
+      # clean up svn:externals
+      externals = rails_env.externals
+      externals.reject!{|n,u| name == n or name == u}
+      rails_env.externals = externals
+    end
   end
 
   def info