[Ada] Remove redundant guard against Any_String

[Pierre-Marie de Rodat via Gcc-patches]

A sequence of checks for a valid Boolean argument fails when applied to
Any_String because its component is not of a Boolean type. The explicit
guard was unnecessary; it was only needed when a First_Index applied to
Any_String would crash, but this was fixed soon after this guard was
added.

Cleanup related to recent fixes for resolution of Boolean operators.

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

gcc/ada/

        * sem_type.adb (Valid_Boolean_Arg): Remove redundant guard.
diff --git a/gcc/ada/sem_type.adb b/gcc/ada/sem_type.adb
--- a/gcc/ada/sem_type.adb
+++ b/gcc/ada/sem_type.adb
@@ -3512,7 +3512,6 @@ package body Sem_Type is
          return True;
 
       elsif Is_Array_Type (T)
-        and then T /= Any_String
         and then Number_Dimensions (T) = 1
         and then Is_Boolean_Type (Component_Type (T))
         and then