Now that we have a color marking scheme for all objects, the
pre-existing 'visited' flag for constants and variables is
redundant: visited is the same as marking an object non-white.

Refactor the respective 'visited' flag logic from constDecl and
varDecl into the color switch in objDecl and remove the 'visited'
flag.

Follow-up on https://go-review.googlesource.com/c/go/+/114517 .

Change-Id: Ie20de65e3b26a5a6ff7b0eddc3d089f56be204e8
Reviewed-on: https://go-review.googlesource.com/115619Reviewed-by: Alan Donovan <adonovan@google.com>