cmd/vet/all: remove skip when x/tools isn't in $GOPATH

Now that the build system has been updated to install x/tools in
$GOPATH (CL 149658), depend on it being there and don't ignore
failures to build the tool.

Update to CL 149097.

Change-Id: I72fde347217533697068b6a6773696cc2b83e9ed
Reviewed-on: https://go-review.googlesource.com/c/150017
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/vet/all/main.go

@@ -236,10 +236,6 @@ func (p platform) vet() {
 	cmd.Stderr = os.Stderr
 	cmd.Stdout = os.Stderr
 	if err := cmd.Run(); err != nil {
-		if _, err := build.Default.Import("golang.org/x/tools/go/analysis/cmd/vet", "", 0); err != nil {
-			fmt.Printf("skipping because golang.org/x/tools is not in GOPATH")
-			return
-		}
 		log.Fatal(err)
 	}