As of commit 79c0173d

 ("[build] Create util/genfsimg for building
filesystem-based images"), the EFI boot file name for each CPU
architecture is defined within the genfsimg script itself, rather than
being passed in as a Makefile parameter.

Remove the now-redundant Makefile definitions for EFI_BOOT_FILE.

Reported-by: Christian I. Nilsson <nikize@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>