gccrs: proc_macro: Remove useless typedef

Initially a typedef was used to easily get the callback function pointer
type. Since this type has changed to a simpler well defined type, this
using declaration has no more purpose.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.cc: Remove type alias.
	* libproc_macro/registration.h: Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>