tree: imx: remove old fit generator script

Since derivatives are moving to binman from usage of the FIT generator
script, and considering the warning introduced in f4a43d29
("Makefile: Warn against using CONFIG_SPL_FIT_GENERATOR"), usage of FIT
generator is discouraged.

Current FIT generator also generates broken output, since commit
3f04db89 ("image: Check for unit addresses in FITs") prohibits using
'@' for unit addresses but the generator script still emits the old
sematics.

Remove the generator script and corresponding call in Makefile, all
derivatives should be migrated to binman in order to provide binary
images.

Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>