[Ada] Remove redundant marking of illegal pragma with error posted

[Ada] Remove redundant marking of illegal pragma with error posted

We flag illegal pragma Elaborate with a call to Error_Msg on the pragma
argument, which in turn calls Set_Error_Posted on the enclosing
statement, i.e. on the pragma itself. The explicit call to
Set_Error_Posted on the pragma itself was redundant.

Cleanup related to handling of illegal code when detecting uninitialized
scalar objects.

gcc/ada/

	* sem_prag.adb (Analyze_Pragma): Remove redundant call to
	Set_Error_Posted.