ipxe: don't override GITVERSION

We had build problems due to the git version checking in the ipxe build
system in the past.  Don't remember the details, but the problem seems
to be gone now, so lets remove the workaround.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

[ most likely ipxe commit 6153c09c41034250408f3596555fcaae715da46c:
  [build] Set GITVERSION only if there is a git repository ]

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
