Remove empty backend.py, update manifest to API v12

Author: knipknap

projector_mode/backend.py

@@ -1,14 +1,13 @@
 name: projector_mode
 display_name: "Projector Mode"
 description: "Display cutting area on external projector/monitor for alignment"
-api_version: 11
+api_version: 12
 depends:
   - rayforge>=0.27.0
 author:
   name: "Rayforge Team"
   email: "noreply@rayforge.org"
 provides:
-  backend: "projector_mode.backend"
   frontend: "projector_mode.frontend"
 license:
   name: "MIT"