sfxge: remove unused common code EFSYS_OPT_RX_HDR_SPLIT

The EFSYS_OPT_RX_HDR_SPLIT optional feature in the common code
implemented the Lookahead Split feature of Windows. This split
received packets at a preconfigured byte offset, and delivered
the header and payload portions to separate receive queues.

Now the common code interface has no callers, so remove it.

Note that this should not be confused with the Header Data Split
feature of Windows, which splits packets at a header boundary.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4888