build: stop testing release-branch.go1.6

Go policy is to support the past two released Go version, which is
currently Go 1.7 and Go 1.8. Most of the Go repos are broken for Go 1.6 now
because everybody is using subtests and contexts.

Previously:
  https://golang.org/cl/31767 removed Go 1.5.
  https://golang.org/cl/20843 removed Go 1.4.

Change-Id: I64b4af609a8be79a0f25a855430b73ba4d442ae0
Reviewed-on: https://go-review.googlesource.com/37466
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

app/build/dash.go

@@ -195,4 +195,5 @@ var gccgoDash = &Dashboard{
 var hiddenBranches = map[string]bool{
 	"release-branch.go1.4": true,
 	"release-branch.go1.5": true,
+	"release-branch.go1.6": true,
 }