The g_clear_pointer macro in GLib gained a bit of type safety when
using GCC and GCC-compatible compilers. It checks if the function
pointer is meant to accept a pointer to the type that the first
argument refers to. Now, GCC doesn't like it if the function pointer is
cast to GDestroyNotify, and emits this warning:
  warning: function called through a non-compatible type

Since the macro, even the earlier version of it, was explicitly
designed to avoid the need to cast the arguments, these redundant casts
should be removed.

glib#1425