Revert: virtio: Support non-legacy network device and queue

And subsequent fix 576b099a.

By adding the mergable header to the vtnet_rx_header structure, the size
was increased by 2 bytes, breaking the alignment of this structure as
described the in preceding comments.

Furthermore, the mergable header does not belong the structure. With the
mergable feature, the header is placed in line with the data, so there is
no need for a separate segment, and misleading to follow the mergable
header with any padding.

The V1 header is effectively identical to mergable header, and the driver
has long supported the mergable feature. Revert this so the later changes
that add V1 support can show how V1 is derived from the existing mergable
buffers support, and to facilitate a later MFC.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27855