Remove source files copied from TensorFlow

The missing symbols are resolved by linking against _pywrap_tensorflow_internal.so,
which was introduced to Horovod with PR horovod#3053.

Signed-off-by: Max H. Gerlach <git@maxgerlach.de>

horovod/tensorflow/CMakeLists.txt

@@ -61,28 +61,6 @@ set(Tensorflow_CXX11 ${Tensorflow_CXX11} PARENT_SCOPE)
 list(APPEND TF_SOURCES "${PROJECT_SOURCE_DIR}/horovod/tensorflow/mpi_ops.cc")
 list(APPEND TF_SOURCES "${PROJECT_SOURCE_DIR}/horovod/tensorflow/xla_mpi_ops.cc")
 
-if(VERSION_DEC GREATER 2006000000 OR VERSION_DEC EQUAL 2006000000)
-    # dependencies for inplace broadcast with resource variables
-    list(APPEND TF_SOURCES "${PROJECT_SOURCE_DIR}/horovod/tensorflow/dense_update_functor.cc"
-                           "${PROJECT_SOURCE_DIR}/horovod/tensorflow/training_op_helpers.cc")
-
-    if(HAVE_CUDA OR HAVE_SUB_PROJECT_CUDA)
-        list(APPEND TF_CUDA_SOURCES "${PROJECT_SOURCE_DIR}/horovod/tensorflow/dense_update_functor_gpu.cc.cu")
-        # dense_update_functor_gpu.cu.cc from TensorFlow source had to be renamed because it was not compiled via nvcc otherwise
-
-        set(CUDA_HOST_COMPILER ${CMAKE_CXX_COMPILER})
-
-        set(ENV{PYTHONPATH} "${PROJECT_SOURCE_DIR}/cmake:$ENV{PYTHONPATH}")
-        execute_process(COMMAND ${PY_EXE} -c "import build_utils; print(' '.join(build_utils.get_nvcc_flags()))"
-                        OUTPUT_VARIABLE HVD_NVCC_COMPILE_FLAGS OUTPUT_STRIP_TRAILING_WHITESPACE)
-
-        list(APPEND CUDA_NVCC_FLAGS "${HVD_NVCC_COMPILE_FLAGS}")
-
-        cuda_add_library(tensorflow_cuda_kernels ${TF_CUDA_SOURCES} STATIC OPTIONS -DGOOGLE_CUDA=1)
-        list(APPEND TF_LINKER_LIBS tensorflow_cuda_kernels)
-    endif()
-endif()
-
 # Create library
 set_output_dir()
 add_library(${TF_TARGET_LIB} SHARED ${SOURCES} ${TF_SOURCES})