main/linux-pam: remove pam_motd from base-session

motd is printed by login utilities (busybox, util-linux-login, or shadow-login)
busybox login does not seem to care about base-session pam, but util-linux and shadow versions will end up printing motd twice with this pam.

Removing pam_motd leaves the motd being displayed exactly once for all three variants.

Fixes #13958 (closed)