systemd: remove various unreferenced files (MR 5070)

Remove OpenRC specific files that were removed for the systemd-specific
forks.

extra-repos/systemd/gdm/gdm.initd

-#!/sbin/openrc-run
-
-supervisor=supervise-daemon
-
-name="Display Manager"
-description="GNOME Display Manager"
-
-command=/usr/sbin/gdm
-command_args="$GDM_OPTS"
-
-depend() {
-	provide display-manager
-	need net dbus
-}

extra-repos/systemd/networkmanager/wwan-no-libsystemd.patch

-Avoid unnecessary dependency on libelogind.
-sd_booted() always returns FALSE on Alpine.
-
---- a/src/core/devices/wwan/nm-modem-manager.c
-+++ b/src/core/devices/wwan/nm-modem-manager.c
-@@ -11,11 +11,8 @@
- 
- #include <libmm-glib.h>
- 
--#if HAVE_LIBSYSTEMD
--#include <systemd/sd-daemon.h>
--#else
-+// XXX-Patched
- #define sd_booted() FALSE
--#endif
- 
- #include "libnm-std-aux/nm-dbus-compat.h"
- #include "nm-modem.h"