Stop adding the stack user to the sudo or wheel group

This is not needed since we explicitly add a sudoers rule for the stack
user.

Change-Id: I4c63ab25811d55b7eee2677c954133dc3e7ae397
diff --git a/stack.sh b/stack.sh
index 71c2f9e..a1f89c1 100755
--- a/stack.sh
+++ b/stack.sh
@@ -145,14 +145,12 @@
     # ability to run sudo
     if [[ "$os_PACKAGE" = "deb" ]]; then
         dpkg -l sudo || apt_get update && install_package sudo
-        STACK_GROUP=sudo
     else
         rpm -qa | grep sudo || install_package sudo
-        STACK_GROUP=wheel
     fi
     if ! getent passwd stack >/dev/null; then
         echo "Creating a user called stack"
-        useradd -U -G $STACK_GROUP -s /bin/bash -d $DEST -m stack
+        useradd -U -s /bin/bash -d $DEST -m stack
     fi
 
     echo "Giving stack user passwordless sudo priviledges"