Let's quote the licence:
« Copyright ©
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
The Software is provided “as is”, without warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose and noninfringement. In no event shall the authors or copyright holders be liable for any claim, damages or other liability, whether in an action of contract, tort or otherwise, arising from, out of or in connection with the software or the use or other dealings in the Software. »
Notice that "the above copyright notice shall be included in all copies". This is the only restriction, really. And this is what forces you legally to give credit.
So, you can even relicence the code of your fork under a more restrictive licence like the GPL or even decide to not distribute your sourcecode at all. However, you cannot remove this limitation: the original licence and copyright claim should still be there (even if you add another licence with additional restrictions).
I must also remind you that:
1) I am not a lawyer, so I may be wrong about this.
2) Parts of Haiku are distributed under various licences, some being more restrictive than this (not our choice, we copied that code from elsewhere). So you have to review all those licences to make sure you comply with all of them - or replace/remove the affected code.