Makefile: Drop SPL_FIT_GENERATOR support

The SPL_FIT_GENERATOR is long superseded by binman, drop SPL_FIT_GENERATOR
support as there are no more users.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>