systemd.bbclass: move disable operation into prerm() script

The 'systemctl disable' operation should not be done in postrm() because
the .service file does not exist anymore and the script will fail hence.

After moving this command into prerm(), postrm() becomes empty and can
be removed completely.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>