[mod] gruntfile: do some polish (no functional change)

- remove unused 'path' constant
- fix indentation
- Put all image tasks at the end, where it is easy to list which image is processed.

Suggested -by: @dalf https://github.com/searxng/searxng/pull/541#discussion_r756247157


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>