Fix the following warning:
NetworkManager[1524461]: Source ID 3844 was not found when attempting to remove it
g_logv (log_domain=0x7f2816fa676e "GLib", log_level=G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7ffe697374d0) at gmessages.c:1391
g_log (log_domain=log_domain@entry=0x7f2816fa676e "GLib", log_level=log_level@entry=G_LOG_LEVEL_CRITICAL, format=format@entry=0x7f2816fae240 "Source ID %u was not found when attempting to remove it") at gmessages.c:1432
g_source_remove (tag=519) at gmain.c:2352
nm_clear_g_source (id=<optimized out>) at ./shared/nm-glib-aux/nm-macros-internal.h:1198
dispose (object=0x55f7289b1ca0) at src/dhcp/nm-dhcp-nettools.c:1433
g_object_unref (_object=<optimized out>) at gobject.c:3303
g_object_unref (_object=0x55f7289b1ca0) at gobject.c:3232
dhcp4_cleanup (self=self@entry=0x55f728af3b20, cleanup_type=cleanup_type@entry=CLEANUP_TYPE_DECONFIGURE, release=release@entry=0) at src/devices/nm-device.c:7565
...Fixes: 45521b1b ('dhcp: nettools: move to failed state if event dispatch fails')