ada: Simplify expansion of negative membership operator

Code cleanup; semantics is unaffected.

gcc/ada/ChangeLog:

	* exp_ch4.adb: (Expand_N_Not_In): Preserve Alternatives in expanded
	membership operator just like preserving Right_Opnd (though only
	one of these fields is present at a time).
	* par-ch4.adb (P_Membership_Test): Remove redundant setting of fields
	to their default values.