[Ada] Simplify checks for library unit pragmas

[Pierre-Marie de Rodat]

If a library unit pragma is ignored then Check_Valid_Library_Unit_Pragma
was rewriting it into a null statement; this null statement then was
then detected by at the callsites to terminate analysis of the pragma.

Now if a pragma is ignored, then Check_Valid_Library_Unit_Pragma raises
an exception and no action further is needed at the callsites.

Cleanup only; behaviour is unaffected.

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

gcc/ada/

	* sem_prag.adb (Check_Valid_Library_Unit_Pragma): Raise
	exception.
	(Analyze_Pragma): Remove detection of rewritten pragmas.
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