go/types, types2: delete TypeList.String

This method is unused and was not discussed in the API
proposals. Note that all error output goes through the
local sprintf which handles arguments specially.

Fixes #50760.

Change-Id: Iae66b0253cc0ece037d3d280951dc2d223c119fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/381634


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>