bhyve nvme: Remove redundant AER Limit checks

The NVMe emulation checked if the Asynchronous Event Request Limit
(a.k.a AERL) would be exceeded in pci_nvme_aer_add(), but this function
is only called from nvme_opc_async_event_req() which also checks for
exceeding the AERL.

(cherry picked from commit 45ab4076f36b428595ba1ce200515fda6af58a76)