xen-netfront: purge page flipping support

Currently neither Linux nor FreeBSD netback supports page flipping. NetBSD
still supports that. It is not sure how many people actually use page
flipping, but page flipping is supposed to be slower than copying nowadays.
It will also shatter frontend / backend address space.

Overall this feature is more of a burden than a benefit.

Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D3888
Sponsored by: Citrix Systems R&D