vmm: Remove an incorrect credential check in vmmdev_open()

Checking pointer equality here is too strict and can lead to incorrect
errors, as credentials are frequently copied to avoid reference counting
overhead.

The check is new with commit 4008758105a6 and was added with the goal of
allowing non-root users to create VMs in mind. Just remove it for now.

Reported by: Alonso Cárdenas Márquez <acardenas@bsd-peru.org>
Reviewed by: jhb
Fixes: 4008758105a6 ("vmm: Validate credentials when opening a vmmdev")
Differential Revision: https://reviews.freebsd.org/D46535