pxe: Drop get_bootfile_path()

This function no longer makes sense, since it is pretty easy to prepend
the boot directory to the filename. Drop it and update its only caller.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Tested-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>