[Misc] clean up remove --inplace. (#6096)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-28-63.ap-northeast-1.compute.internal>

docs/source/contribute.rst

@@ -202,7 +202,7 @@ If you are working on performance critical part, you may want to turn on Cython
 .. code-block:: bash
 
    cd python
-   python setup.py build_ext --inplace
+   python setup.py build_ext
 
 You could test the build by running the following command and see the path of your local clone.
 

tests/scripts/build_dgl.sh

@@ -45,8 +45,8 @@ if [[ $1 == "cugraph" ]]; then
     pip uninstall -y dgl
     # test install
     python3 setup.py install
-    # test inplace build (for cython)
-    python3 setup.py build_ext --inplace
+    # test build (for cython)
+    python3 setup.py build_ext
 else
     for backend in pytorch mxnet tensorflow
     do
@@ -55,8 +55,8 @@ else
     pip uninstall -y dgl
     # test install
     python3 setup.py install
-    # test inplace build (for cython)
-    python3 setup.py build_ext --inplace
+    # test build (for cython)
+    python3 setup.py build_ext
     done
 fi
 popd