go/types: remove unused pathString and rename objPathString to pathString (cleanup)
go/types: remove unused pathString and rename objPathString to pathString (cleanup)

This eliminates an old TODO.

Change-Id: I36d666905f43252f5d338b11ef9c1ed8b5f22b1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/195817
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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