xuser-account: remove GROUPADD_PARAM
The group shutdown had been provided by base-passwd, otherwise, it may cause
sysvinit failed to build:
$ bitbake xuser-account sysvinit && bitbake xuser-account sysvinit -ccleansstate && bitbake sysvinit
[snip]
| chown: invalid user: root.shutdown
[snip]
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>