[Ada] Cleanup in error about unreachable code

[Pierre-Marie de Rodat via Gcc-patches]

Cleanup only; behaviour is unaffected.

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

gcc/ada/

        * sem_ch5.adb (Check_Unreachable_Code): Avoid explicit use of
        Sloc; this should also help when we finally use Source_Span for
        prettier error messages.
diff --git a/gcc/ada/sem_ch5.adb b/gcc/ada/sem_ch5.adb
--- a/gcc/ada/sem_ch5.adb
+++ b/gcc/ada/sem_ch5.adb
@@ -4468,8 +4468,7 @@ package body Sem_Ch5 is
                   end loop;
                end if;
 
-               Error_Msg
-                 ("??unreachable code!", Sloc (Error_Node), Error_Node);
+               Error_Msg_N ("??unreachable code!", Error_Node);
             end if;
 
          --  If the unconditional transfer of control instruction is the