[ImperialCrown]

Remove the nut on the other side of the bolt and tap it out of the hole. I believe that the bolt is splined into the hole and the reason for the odd pentagonal head is to prevent folks from trying to force it with a wrench and subsequently shear it off.

 

[AJ_Schreiber]

I felt the other side of the bolt and it feels like a carriage bolt. I've never seen a 5 sided bolt before.

 

[AJ_Schreiber]

To follow up and answer anyone in the future:
I ended up just grinding the 5 sided bolt off. The nut is attached to a carriage bolt so you can not loosen the other end. After grinding the Penta-bolt off I bought a 1/2" by 3" carriage bolt and nut from Lowes to reinstall the new mount and it works perfectly. DO NOT grind on the head of the carriage bolt itself, you need to preserve that bracket to transfer over to the new mount. Hope this helps

 

[NYBo]

I hope you used a hardened carriage bolt!

 

[AJ_Schreiber]

used the highest grade I could find