(Ada) Cleanup code by using ada_is_access_to_unconstrained_array call.

This patch just avoids code duplication by using a function we
introduced recently (ada_is_access_to_unconstrained_array).

gdb/ChangeLog:

    * ada-lang.c (ada_is_access_to_unconstrained_array): Remove static
    declaration.
    * ada-lang.h: add ada_is_access_to_unconstrained_array prototype.
    * ada-varobj.c (ada_varobj_get_number_of_children,
    ada_varobj_describe_child, ada_value_is_changeable_p): Cleanup code.

Tested on x86_64-linux.
No new testcase provided, as this is just a refactoring.