[Doc][Installation] delete python setup.py develop (vllm-project#3989)

Author: youkaichao

docs/source/getting_started/installation.rst

@@ -85,13 +85,3 @@ You can also build and install vLLM from source:
 
         $ nvcc --version # verify that nvcc is in your PATH
         $ ${CUDA_HOME}/bin/nvcc --version # verify that nvcc is in your CUDA_HOME
-
-.. note::
-    If you are developing the C++ backend of vLLM, consider building vLLM with
-
-    .. code-block:: console
-
-        $ python setup.py develop
-
-    since it will give you incremental builds. The downside is that this method
-    is `deprecated by setuptools <https://github.com/pypa/setuptools/issues/917>`_.