go/types: remove a test case and update comment
go/types: remove a test case and update comment

The original need for the extra test case and issue was eliminated
by https://golang.org/cl/116815 which introduced systematic cycle
detection. Now that we correctly report the cycle, we can't say much
about the invalid cast anyway (the type is invalid due to the cycle).

A more sophisticated approach would be able to tell the size of
a function type independent of the details of that type, but the
type-checker is not set up for this kind of lazy type-checking.

Fixes #23127.

Change-Id: Ia8479e66baf630ce96f6f36770c8e1c810c59ddc
Reviewed-on: https://go-review.googlesource.com/c/141640
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

akutz/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,cloudflare/go,nawawi/go,golang/go,akutz/go,nawawi/go,josharian/go,golang/go,CAFxX/go,akutz/go,akutz/go,josharian/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,golang/go,golang/go,CAFxX/go,golang/go,akutz/go,nawawi/go,CAFxX/go,josharian/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go