ixl(4): Remove iavf(4) source files

Since iavf(4) no longer shares code with ixl(4) as of commit
f2fbd56a8d07665bc0a5e8b7e40026b50a591e2a and now has its own directory,
remove these now-unused iavf(4)-only files.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D28638

(cherry picked from commit 409b36ad911d0a12a45c1488369b458965691379)