Makefile: Drop SPL_FIT_SOURCE support

The SPL_FIT_SOURCE is long superseded by SPL_FIT_GENERATOR which
is long superseded by binman, drop SPL_FIT_SOURCE support as there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

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