Remove preprocessing of CFLAGS, CPPFLAGS, and LDFLAGS in Python scripts. (pytorch#23528)

Summary:
After pytorch#23455, there is no need of this preprocessing in Python scripts.
They will be automatically processed in CMake (plus CPPFLAGS here
probably meant to be CXXFLAGS).

Reference:

- https://cmake.org/cmake/help/v3.15/envvar/CFLAGS.html
- https://cmake.org/cmake/help/v3.15/envvar/CXXFLAGS.html
- https://cmake.org/cmake/help/v3.15/envvar/LDFLAGS.html
Pull Request resolved: pytorch#23528

Differential Revision: D16561561

Pulled By: ezyang

fbshipit-source-id: 962a27a2b0a18db0f95477ad067a2611e4128187

Author: xuhdev

tools/setup_helpers/cmake.py

@@ -187,9 +187,6 @@ def generate(self, version, cmake_python_library, build_python, build_test, my_e
                 toolset_expr = ','.join(["{}={}".format(k, v) for k, v in toolset_dict.items()])
                 args.append('-T' + toolset_expr)
 
-        cflags = os.getenv('CFLAGS', "") + " " + os.getenv('CPPFLAGS', "")
-        ldflags = os.getenv('LDFLAGS', "")
-
         base_dir = os.path.dirname(os.path.dirname(os.path.dirname(
             os.path.abspath(__file__))))
         install_dir = os.path.join(base_dir, "torch")
@@ -259,10 +256,6 @@ def generate(self, version, cmake_python_library, build_python, build_test, my_e
         # Options starting with CMAKE_
         cmake__options = {
             'CMAKE_INSTALL_PREFIX': install_dir,
-            'CMAKE_C_FLAGS': cflags,
-            'CMAKE_CXX_FLAGS': cflags,
-            'CMAKE_EXE_LINKER_FLAGS': ldflags,
-            'CMAKE_SHARED_LINKER_FLAGS': ldflags,
         }
 
         # We set some CMAKE_* options in our Python build code instead of relying on the user's direct settings. Emit an