Remove adjust_type_signedness

I happened across adjust_type_signedness, which may be used to modify
a type when printing an Ada value.  Modifying a type like this is a
bad idea -- they should normally be considered immutable.  Removing
this function still passes both the dejagnu and internal AdaCore
tests, though, so this patch drops it.

As this was reviewed internally, and only affect Ada, I am checking it
in.

gdb/ada-valprint.c

@@ -37,15 +37,6 @@ static int print_field_values (struct value *, struct value *,
 
 
 
-/* Make TYPE unsigned if its range of values includes no negatives.  */
-static void
-adjust_type_signedness (struct type *type)
-{
-  if (type != NULL && type->code () == TYPE_CODE_RANGE
-      && type->bounds ()->low.const_val () >= 0)
-    type->set_is_unsigned (true);
-}
-
 /* Assuming TYPE is a simple array type, prints its lower bound on STREAM,
    if non-standard (i.e., other than 1 for numbers, other than lower bound
    of index type for enumerated type).  Returns 1 if something printed,
@@ -652,7 +643,6 @@ print_field_values (struct value *value, struct value *outer_value,
 	      int bit_size = TYPE_FIELD_BITSIZE (type, i);
 	      struct value_print_options opts;
 
-	      adjust_type_signedness (type->field (i).type ());
 	      v = ada_value_primitive_packed_val
 		    (value, nullptr,
 		     bit_pos / HOST_CHAR_BIT,