remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/core/dbus-socket.c

@@ -21,9 +21,7 @@
 
 #include "unit.h"
 #include "socket.h"
-#include "dbus-unit.h"
 #include "dbus-execute.h"
-#include "dbus-kill.h"
 #include "dbus-cgroup.h"
 #include "dbus-socket.h"
 #include "bus-util.h"