Makefile: Drop GO15VENDOREXPERIMENT

This became the default behavior in Go 1.6 and the switch was dropped
in Go 1.7 [1].  We've required Go 1.8+ since at least 5538e7c (Update
development.md to make it clear that only Go version 1.8 is supported
now, 2017-11-16, #775), so there's no need to set the switch anymore.

[1]: https://blog.golang.org/go1.7

Author: wking

Makefile

@@ -1,4 +1,3 @@
-export GO15VENDOREXPERIMENT:=1
 export CGO_ENABLED:=0
 export GOARCH:=amd64
 export PATH:=$(PATH):$(PWD)