image: Drop IMAGE_ENABLE_BEST_MATCH

This is not needed with Kconfig, since we can use IS_ENABLED() easily
enough. Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>