go: remove extra GOROOT env check

Some scripts are setting GOROOT. They are setting it to the
right thing, so it's not serious, and removing this check
is the fastest way to fix the build.

Change-Id: I40b1406a3a05b205061c4ef78bf7446214a1d4f9
diff --git a/go b/go
index fde0ef6..10a450b 100755
--- a/go
+++ b/go
@@ -22,11 +22,6 @@
     ;;
 esac
 
-if [[ "$GOROOT" != "" ]]; then
-    echo "buildtools/go: go selection script called with GOROOT set"
-    exit 1
-fi
-
 # Setting GOROOT is a workaround for https://golang.org/issue/18678.
 # Remove this (and switch to exec_tool.sh) when Go 1.9 is released.
 export GOROOT="$SCRIPT_ROOT/$HOST_PLATFORM/go"