Simplify ada_lookup_struct_elt_type

This patch simplifies ada_lookup_struct_elt_type by changing it to
call find_struct_field.  The two functions were substantially similar,
even to the point of having identical comments.

I tested this using both the gdb test suite and the internal AdaCore
test suite.  Given this and the fact that it is Ada-specific, I am
checking it in.