Convert config/chroot_local-hooks/21-gdm_unit_file to a drop-in override.

I see no reason to patch it at build time the way we did it.

Will-Fix: #10606

config/chroot_local-hooks/21-gdm_unit_file → config/chroot_local-includes/lib/systemd/system/gdm.service.d/restart.conf

-#!/bin/sh
-
-set -e
-
-echo "Making GDM *not* restart automatically"
+[Service]
 
 # The GDM unit file has a Restart=always directive, which is good in the
 # general case. However, it breaks our emergency shutdown on boot medium
-# removal feature. Let's disable it, then.
-
-perl -pi -E 's{^Restart=.*$}{Restart=no}' /lib/systemd/system/gdm.service
+# removal feature, so we disable it.
+Restart=no

wiki/src/contribute/design/memory_erasure.mdwn

@@ -92,7 +92,7 @@ physically removed.
 - [[!tails_gitweb config/chroot_local-includes/lib/systemd/system/tails-sdmem-on-media-removal.service]]
 - [[!tails_gitweb config/chroot_local-hooks/52-update-rc.d]]
 - [[!tails_gitweb config/chroot_local-patches/run_kexec-load_even_in_emergency_shutdown.diff]]
-- [[!tails_gitweb config/chroot_local-hooks/21-gdm_unit_file]]
+- [[!tails_gitweb config/chroot_local-includes/lib/systemd/system/gdm.service.d/restart.conf]]
 
 #### Making sure needed files are available