fix(optimize): stop Dock Refresh from deleting wallpaper database

opt_dock_refresh ran `find "$HOME/Library/Application Support/Dock"
-name "*.db"` and unlinked every match. desktoppicture.db is macOS's
desktop wallpaper database; deleting it caused the wallpaper to reset
to the macOS default after the Dock restart.

No known .db under that directory is a Dock cache that must be cleared
to refresh: killall Dock plus touching com.apple.dock.plist already
handles the actual refresh. Drop the .db deletion branch entirely.

Closes #995.