[Ada] Minimize parts of Ada.Strings.Fixed marked SPARK_Mode => Off

gcc/ada/

	* libgnat/a-strfix.adb (Delete, Insert, Overwrite,
	Replace_Slice): Remove SPARK_Mode Off.
	* libgnat/a-strfix.ads (Insert, Overwrite, Replace_Slice):
	Strengthen precondition.
2 files changed