Remove other extensions' source files from EXTENSION_TRAINING_SRCS and TRAIN_XOR_SRCS (#13433)

Looks like this was an artifact of improperly specified deps in (now
deleted) cmake_deps.toml. extension/training/CMakeLists.txt already has
the relevant dependencies, so we should be able to just remove these.

Author: swolchok

shim_et/xplat/executorch/build/build_variables.bzl

@@ -370,27 +370,14 @@ THREADPOOL_SRCS = [
 EXTENSION_THREADPOOL_SRCS = ["extension/threadpool/" + x for x in THREADPOOL_SRCS]
 
 EXTENSION_TRAINING_SRCS = [
-    "extension/data_loader/file_data_loader.cpp",
-    "extension/data_loader/mmap_data_loader.cpp",
-    "extension/flat_tensor/flat_tensor_data_map.cpp",
-    "extension/flat_tensor/serialize/flat_tensor_header.cpp",
-    "extension/module/module.cpp",
     "extension/training/module/training_module.cpp",
     "extension/training/optimizer/sgd.cpp",
 ]
 
 TRAIN_XOR_SRCS = [
-    "extension/data_loader/file_data_loader.cpp",
-    "extension/data_loader/mmap_data_loader.cpp",
-    "extension/flat_tensor/flat_tensor_data_map.cpp",
-    "extension/flat_tensor/serialize/flat_tensor_header.cpp",
+    # REVIEW: removing this breaks the build; where is it supposed to come from?
     "extension/flat_tensor/serialize/serialize.cpp",
-    "extension/module/module.cpp",
-    "extension/tensor/tensor_ptr.cpp",
-    "extension/tensor/tensor_ptr_maker.cpp",
     "extension/training/examples/XOR/train.cpp",
-    "extension/training/module/training_module.cpp",
-    "extension/training/optimizer/sgd.cpp",
 ]
 
 EXECUTOR_RUNNER_SRCS = [