lib,ospfd,ospf6d: remove duplicated function

lib,ospfd,ospf6d: remove duplicated function

Move `is_default_prefix` variations to `lib/prefix.h` and make the code
use the library version instead of implementing it again.

NOTE
----

The function was split into per family versions to cover all types.
Using `union prefixconstptr` is not possible due to static analyzer
warnings which cause CI to fail.

The specific cases that would cause this failure were:
 - Caller used `struct prefix_ipv4` and called the generic function.
 - `is_default_prefix` with signature using `const struct prefix *` or
   `union prefixconstptr`.

The compiler would complain about reading bytes outside of the memory
bounds even though it did not take into account the `prefix->family`
part.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>