go/types, types2: remove InvalidTypeCycle from literals.go

go/types, types2: remove InvalidTypeCycle from literals.go

Both CL 722161 and CL 724140 implement a more general solution to
detecting cycles involving values of a type on the object path.

The logic in literals.go was intended to be a stop-gap solution and
is no longer necessary.

Change-Id: I328c0febf35444f07fc1894278dc76ab140710bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/724380
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>