Don't copy presets/isles.png

[muzikbike]

This file is actually completely unused by the game (as a leftover from 1.8-1.12.2's customizable world system), so having it be migrated is ultimately pointless. This can easily be fixed by removing the corresponding line from the 1.20.2 main java file.

Bug report on Mojira: https://bugs.mojang.com/browse/MC-269534