[Ada] Remove comment about a long gone formal verification mode

[Pierre-Marie de Rodat via Gcc-patches]

Remove outdated a comment about the very first SPARK experiments
in GNAT.

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

gcc/ada/

        * sem_ch6.adb (Check_Missing_Return): Remove outdated comment.
diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -2987,9 +2987,7 @@ package body Sem_Ch6 is
 
       procedure Check_Missing_Return;
       --  Checks for a function with a no return statements, and also performs
-      --  the warning checks implemented by Check_Returns. In formal mode, also
-      --  verify that a function ends with a RETURN and that a procedure does
-      --  not contain any RETURN.
+      --  the warning checks implemented by Check_Returns.
 
       function Disambiguate_Spec return Entity_Id;
       --  When a primitive is declared between the private view and the full