Remove zll* source files (#15603)

* Remove zll* files. They seem unused

* Also remove references to headers from the build system. Nobody includes them

Author: andy31415

src/app/chip_data_model.gni

@@ -83,9 +83,6 @@ template("chip_data_model") {
       "${_app_root}/clusters/on-off-server/on-off-server.h",
       "${_app_root}/clusters/scenes/scenes-tokens.h",
       "${_app_root}/clusters/scenes/scenes.h",
-      "${_app_root}/clusters/zll-level-control-server/zll-level-control-server.h",
-      "${_app_root}/clusters/zll-on-off-server/zll-on-off-server.h",
-      "${_app_root}/clusters/zll-scenes-server/zll-scenes-server.h",
       "${_app_root}/reporting/reporting.h",
       "${_app_root}/util/DataModelHandler.cpp",
       "${_app_root}/util/af-event.cpp",