[#5404] Undef Kernel.open before creating open scope

[garethrees]

All Ruby objects inherit from Object, which includes the Kernel module,
which in turn, has an open method. Redefining .open was creating lots
of WARN level messages in the logs:

W, [2019-10-10T14:17:40.179390 #30917]  WARN -- : Creating scope
:open. Overwriting existing method PublicBodyChangeRequest.open

Since we're not using Kernel.open, we can explicitly undefine it prior
to defining our own .open in the form of a scope.

Fixes #5405.

[garethrees]

Blocked by #5222/#5223, will be mergeable after #6052.

[gbp]

Nice, I always wondered why these were appearing.

I don't really understand why we need to call undef_method on the singleton_class instead of calling as a class method but this certainly works.

[garethrees]

I don't really understand why we need to call undef_method on the singleton_class instead of calling as a class method but this certainly works.

Ah, I left some earlier investigation in #5405 (comment). I don't fully understand it either tbh. I assume it's something to do with being explicit about removing the open method of PublicBodyChangeRequest rather than the open of Kernel.