[Ada] Simplify detection of delayed aggregates

[Pierre-Marie de Rodat]

Replace IF with a single RETURN statement. Cleanup related to expansion
of aggregates in GNATprove mode; semantics is unaffected.

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

gcc/ada/

	* exp_aggr.adb (Is_Delayed_Aggregate): Simplify.
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