Remove COMPILE_CUSTOM_KERNELS env (#4864)

With more and more custom ops merged, disable `COMPILE_CUSTOM_KERNELS `
for vllm ascend seems useless now. Let's enable csrc compile by default.

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

docs/source/installation.md

@@ -162,12 +162,10 @@ pip install -v -e .
 cd ..
 ```
 
-vllm-ascend will build custom operators by default. If you don't want to build it, set `COMPILE_CUSTOM_KERNELS=0` environment to disable it.
 If you are building custom operators for Atlas A3, you should run `git submodule update --init --recursive` manually, or ensure your environment has Internet access.
 :::
 
 ```{note}
-If you are building from v0.7.3-dev and intend to use sleep mode feature, you should set `COMPILE_CUSTOM_KERNELS=1` manually.
 To build custom operators, gcc/g++ higher than 8 and c++ 17 or higher is required. If you're using `pip install -e .` and encounter a torch-npu version conflict, please install with `pip install --no-build-isolation -e .` to build on system env.
 If you encounter other problems during compiling, it is probably because unexpected compiler is being used, you may export `CXX_COMPILER` and `C_COMPILER` in environment to specify your g++ and gcc locations before compiling.
 ```