Remove adjust_type_signedness

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.