libpam: remove unused code

from commit b0384720a46fb25c4ad180e3f256ffdeb53dc8a6,
which upgrade libpam to 1.5.1, packaging is adjustd,
and the binary is packaged into libpam-runtime, so we
don't need to append them to pam-plugin-xxx.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>