libpam: remove MLPREFIX from PN

libpam: remove MLPREFIX from PN

The commit df3038768f59f7a0c814974ff674d4e59cbdfca4 changed 'libpam' to
'pn', then we don't need the "MLPREFIX + pn" any more, otherwise we
would get the name like: "lib32-lib32-libpam-x", and the warn:

WARNING: QA Issue: lib32-pam-plugin-access rdepends on
lib32-lib32-libpam-suffix, but it isn't a build dependency? [build-deps]

(From OE-Core rev: 804c1284891c2654e1431fe4d777ca41ac466120)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>