autogen: drop unnecessary -Wno-gnu for the clang shortcut

It is no longer necessary after 65e3a2cf7c3b399853dd309f702ca5078b7d16ea
where -Wno-gnu-variable-sized-type-not-at-end is added if we detect
that the compiler is clang.
pam_systemd: remove unused null check

username was already checked with isempty() and cannot be null at
this point.

CID#1237766