Remove last mention of GO15VENDOREXPERIMENT
Since #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]
[2]
# 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
