libpam: Remove option 'obscure' from common-password

libpam: Remove option 'obscure' from common-password

libpam does not support 'obscure' checks to password,
there are the same checks in pam_cracklib module.
And this fix can remove the below error message while
updating password with 'passwd':
pam_unix(passwd:chauthtok):unrecognized option[obscure]

Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>