Cleanup related to expansion of dispatching wrappers for GNATprove,
which just like instantiation involves copying of a specification from
an existing subprogram.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch12.adb (Instantiate_Formal_Subprogram): Remove redundant
	call to Set_Defining_Unit_Name; a similar call is done few lines
	below.