MM-23817: Use auto-vendoring (#14231)

With Go 1.14 (https://tip.golang.org/doc/go1.14#go-command),
it now automatically checks for the existence of a vendor directory.

So there is no need to explicitly set the `-mod=vendor` flag.

Makefile

@@ -52,7 +52,7 @@ else
 endif
 
 # Go Flags
-GOFLAGS ?= $(GOFLAGS:) -mod=vendor
+GOFLAGS ?= $(GOFLAGS:)
 # We need to export GOBIN to allow it to be set
 # for processes spawned from the Makefile
 export GOBIN ?= $(PWD)/bin