Remove unnecessary [NoInterfaceObject] from testing IDL files

Testing IDL files no longer need [NoInterfaceObject],
as we now don't feed them into the list of main interface files
('main_interface_idl_files' in generated_bindings.gyp);
previously it was necessary because we didn't distinguish these files.

This removes the extended attribute, simplifying them.

TBR=haraken
BUG=341748

Review URL: https://codereview.chromium.org/205943003

git-svn-id: svn://svn.chromium.org/blink/trunk@169629 bbb929c8-8fbe-4397-9dbb-9b2b20218538