Check for the expression of the aspect Static were done inside branches
corresponding to inside-a-generic and outside-a-generic. Now this check
occurs only once. Code cleanup; behaviour is unaffected.

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

gcc/ada/

	* sem_ch13.adb (Analyze_Aspect_Static): Refactor to have a
	single check for the expression being present; adapt comments.