refactor: remove legacy .lpschem format (TODO 117)

- Delete schematic/legacy/lpschem/ (LPSchematicLegacy + 5 pojo Gson models)
- Delete schematic/convert/LpschemConverter and /lp schematic convert command
- Remove SchematicClipboardBuilder.fromLegacy, convert branches in dispatcher/
  tab-completer/help, locales (ru/en), SCHEMATICS.md and yml comments
- Kept only .schem (WorldEdit) + .nbt
- Build green: checkstyle 0 errors, PMD 33

src/main/resources/locales/en.yml

@@ -354,7 +354,6 @@ schematic:
     line_pos2: "&#FF4444/LoParkour schematic pos2 &#404040- &#A0A0A0Set position 2"
     line_create: "&#FF4444/LoParkour create <difficulty> &#404040- &#A0A0A0Save selection to .schem + yml"
     line_create_named: "&#FF4444/LoParkour schematic create <name> <difficulty> &#404040- &#A0A0A0Named save"
-    line_convert: "&#FF4444/LoParkour schematic convert &#404040- &#A0A0A0Convert .lpschem to .schem"
     line_paste: "&#FF4444/LoParkour schematic paste <name> &#404040- &#A0A0A0Test-paste"
     line_list: "&#FF4444/LoParkour schematic list &#404040- &#A0A0A0List loaded"
     line_reload: "&#FF4444/LoParkour schematic reload &#404040- &#A0A0A0Reload from disk"
@@ -370,8 +369,6 @@ schematic:
   selection_incomplete: "&#FF4444Selection incomplete. Use wand or pos1/pos2 first."
   created: "&#A0A0A0Added &#FF4444%s &#404040(diff %s) as &#FF4444%s"
   create_failed: "&#FF4444Create failed: %s"
-  convert_done: "&#A0A0A0Converted %s file(s), %s failed."
-  convert_truncated: "&#A0A0A0...and %s more (see console)."
   not_found: "&#FF4444Schematic '%s' not found."
   pasted: "&#A0A0A0Pasted &#FF4444%s&#A0A0A0."