arm64 nexus: remove incorrect warning

acpi_config_intr() will be called when an arm64 system booted with ACPI.
We do the interrupt mapping for ACPI interrupts in nexus_acpi_map_intr()
on arm64, so acpi_config_intr() has to just return success without
printing this error message.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D19432