pxe-network-installer: remove gpxelinux.0

pxe-network-installer: remove gpxelinux.0

gpxe has been removed from syslinux 4 years ago:
==================================================
commit f2f897a1762fab84d2905f32b1c15dd7b42abb56
Author: H. Peter Anvin <hpa@zytor.com>
Date:   Tue Feb 9 18:08:47 2016 -0800

    gpxe: delete long since obsolete snapshot of gPXE

    gPXE has been deprecated in favor of iPXE for many, many years now.
    It is much better than users get it directly from the iPXE project,
    since we should no longer need any special modifications for Syslinux
    use.

    Signed-off-by: H. Peter Anvin <hpa@zytor.com>
==================================================

Remove it for now since it doesn't seem to be mandatory and
we can use pxelinux.0 for now.

Issue-ID: INF-140
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Change-Id: I7dcf6d1de1496a470130a4b43aef50159947c6f3