remove unimplemented is_udt function declaration from prelude

This caused linking issues for me.

There previously was an implementation, but this was removed in the SACarg rewrite: 6a57728c . So this function has not worked since march 2025.

If instead we need it re-implemented, the previous implementation was:

bool
SACARGisUdt (basetype btype, SACarg *arg)
{
    return (SACARG_BTYPE (arg) == btype);
}

But I dont know if that still works.

Fixes #2553