Remove last mention of GO15VENDOREXPERIMENT

Since prometheus#2750[1] prometheus depends on go 1.8. go 1.7 enabled vendoring by
default and removes the corresponding variable[2], simplifying things a bit.

[1] prometheus#2750
[2] https://blog.golang.org/go1.7

Makefile

@@ -11,7 +11,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-GO           ?= GO15VENDOREXPERIMENT=1 go
+GO           ?= go
 GOFMT        ?= $(GO)fmt
 FIRST_GOPATH := $(firstword $(subst :, ,$(shell $(GO) env GOPATH)))
 PROMU        := $(FIRST_GOPATH)/bin/promu