Revert "busybox: added linux-pam support"

This reverts commit 083aeac9.

Commit f21b2558 (busybox: added linux-pam support, 2015-06-02) added
linux-pam support already, no need to do this again. Commit 083aeac9
(busybox: added linux-pam support, 2015-07-14) also effectively annulled the
build failure fix from commit 98e1848d (busybox: PAM support needs thread
support, 2015-07-04). Revert it.

Fixes:
http://autobuild.buildroot.net/results/8df/8df14bde19a41b78aa6422a1649a570914f747ee/
http://autobuild.buildroot.net/results/30d/30dd11b576b590de513031fce5e034353f2f5544/
http://autobuild.buildroot.net/results/de6/de628d84b0ed49be8f5d021c789357b2e66f2b02/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>