Fix corals and kelps duping with mesecons' sticky piston

[zmv7]

This pull request fixes a glitch/exploit that could be used to dupe default:coral_skeleton and default:sand nodes infinitely.

Before patch

Place mesecons' sticky piston next to cyan/green/pink coral or default:sand_with_kelp and switch the piston couple times. In the result, extra default:coral_skeleton or default:sand node will appear on each piston retraction.

After patch

The cyan/green/pink corals and default:sand_with_kelp are moving by piston normally, without creating extra nodes.

[appgurueu]

after_destruct is called by minetest.remove_node. This is indeed wrong - when a mod wants to remove a node, that node shouldn't turn into sand; it is reasonable for the sand and the kelp attached to it to be gone.

Using after_dig_node instead makes more sense. You don't have to add unused parameters though.

[sfan5]

LGTM