backuptool: Do not set ADDOND_VERSION=2 for a-only

There are addon.d scripts that rely on the value of ADDOND_VERSION
to determine if they're being called from a-only vs a/b backuptool.

If they declare ADDOND_VERSION=3, they shall stop doing that;
otherwise offer them the same environment, that is unset ADDOND_VERSION
for a-only backuptool.

Change-Id: I1be21eda2e6ec9837b3080bb5e7fbe5241318eaa