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]

(From OE-Core rev: f5b90eeed7366432b39c7cd8c6ee8c23e2d4abe8)

Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ea761db)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/pam/libpam/pam.d/common-password

@@ -10,13 +10,10 @@
 # The "sha512" option enables salted SHA512 passwords.  Without this option,
 # the default is Unix crypt.  Prior releases used the option "md5".
 #
-# The "obscure" option replaces the old `OBSCURE_CHECKS_ENAB' option in
-# login.defs.
-#
 # See the pam_unix manpage for other options.
 
 # here are the per-package modules (the "Primary" block)
-password	[success=1 default=ignore]	pam_unix.so obscure sha512
+password	[success=1 default=ignore]	pam_unix.so sha512
 # here's the fallback if no module succeeds
 password	requisite			pam_deny.so
 # prime the stack with a positive return value if there isn't one already;