Remove duplicate images, legacy solutions, and fix prettierignore path

- Remove unreferenced duplicate images from side_quests/img/ (the real
  copies are in plugin_development/img/)
- Remove legacy solution files from side-quests/solutions/plugin_development/
  (leftover from an earlier iteration; current solutions are in
  side-quests/plugin_development/solutions/)
- Fix .prettierignore COPYING path from hello-plugins/ to
  side-quests/plugin_development/

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pinin4fjords

.prettierignore

@@ -17,4 +17,4 @@ side-quests/solutions/nf-core/**/*.yml
 hello-nf-core/solutions/core-hello-*/**
 
 # Ignore auto-generated license files in plugin solutions
-hello-plugins/**/COPYING
+side-quests/plugin_development/**/COPYING