imx: spl_imx_romapi.c: remove dead code

These IS_ENABLED(CONFIG_SPL_LOAD_FIT) cases can no longer be reached,
and thus get_fit_image_size() is also redundant.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Fabio Estevam <festevam@denx.de>