cmd/buildlet: remove unused reverseModeBuildKey var

Change-Id: I774651dc1769352f11cf13d15c3f5a8a11a537e3
Reviewed-on: https://go-review.googlesource.com/34975
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/buildlet/reverse.go b/cmd/buildlet/reverse.go
index 9302738..67f6af7 100644
--- a/cmd/buildlet/reverse.go
+++ b/cmd/buildlet/reverse.go
@@ -28,8 +28,6 @@
 	"golang.org/x/build/revdial"
 )
 
-var reverseModeBuildKey string
-
 // mode is either a BuildConfig or HostConfig name (map key in x/build/dashboard/builders.go)
 func keyForMode(mode string) (string, error) {
 	if isDevReverseMode() {