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.

Reviewed by: imp, allanjude
Tested by: jason@tubnor.net
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33570