fastboot: sparse: remove redundant argument to write_sparse_image

'sz' has no use for write_sparse_image, remove it simplifying the api.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

include/image-sparse.h

@@ -36,4 +36,4 @@ static inline int is_sparse_image(void *buf)
 }
 
 void write_sparse_image(struct sparse_storage *info, const char *part_name,
-			void *data, unsigned sz);
+			void *data);