doas: don't configure pamdir

In the future, doas won't ship PAM files (see
https://github.com/Duncaen/OpenDoas/commit/cfa9f0d3b306d6c1287ec4f2aa42be29de66c9de),
and we already configure PAM in the doas module. Configuring the pamdir
serves no purpose.

pkgs/tools/security/doas/default.nix

···
   configureFlags = [
     (lib.optionalString withTimestamp "--with-timestamp") # to allow the "persist" setting
     (lib.optionalString (!withPAM) "--without-pam")
-    "--pamdir=${placeholder "out"}/etc/pam.d"
   ];
 
   patches = [