cmd/gomobile: fix tests on builders

The naïve string matching of GOROOT fails on the builders where
GOROOT is a string prefix og GOPATH.

We're not manipulating GOROOT in the tools anymore so do the easiest
thing and delete GOROOT matching.

Change-Id: I194883b9c25f38e2bfdd3eb0ccc9fade76f45549
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/165579
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/gomobile/build.go b/cmd/gomobile/build.go
index 45f950a..6706cf6 100644
--- a/cmd/gomobile/build.go
+++ b/cmd/gomobile/build.go
@@ -207,9 +207,6 @@
 	if gomobilepath != "" {
 		cmd = strings.Replace(cmd, gomobilepath, "$GOMOBILE", -1)
 	}
-	if goroot := goEnv("GOROOT"); goroot != "" {
-		cmd = strings.Replace(cmd, goroot, "$GOROOT", -1)
-	}
 	if gopath := goEnv("GOPATH"); gopath != "" {
 		cmd = strings.Replace(cmd, gopath, "$GOPATH", -1)
 	}