Revert r318180 and re-enable AIO tests on md(4) by default.

The 'physio' fast-path used by AIO requests on md(4) devices, is not
gated on the unsafe_aio knob. Prior to r327755, some AIO requests could
fail the fast-path and fall back to the slow-path (requests for devices
not supporting unmapped I/O and requests which failed with EFAULT during
the fast-path). However, those cases now return a suitable error rather
than using the slow-path.

PR: 217261
Reviewed by: asomers
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D14742