ada: Remove obsolete code from Safe_Unchecked_Type_Conversion

That's a kludge added to work around the limitations of the stack checking
mechanism used in the early days.

gcc/ada/ChangeLog:

	* exp_util.ads (May_Generate_Large_Temp): Delete.
	* exp_util.adb (May_Generate_Large_Temp): Likewise.
	(Safe_Unchecked_Type_Conversion): Do not take stack checking into
	account to compute the result.