[Ada] Cleanup copying of parameter lists

[Pierre-Marie de Rodat]

Cleanup related to expansion of predefined equality for GNATprove;
semantics is unaffected.

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

gcc/ada/

	* sem_util.adb (Copy_Parameter_List): Refactor to remove
	repeated calls to First_Formal.
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