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>

Dockerfile.a3

@@ -18,14 +18,12 @@
 FROM quay.io/ascend/cann:8.3.rc2-a3-ubuntu22.04-py3.11
 
 ARG PIP_INDEX_URL="https://mirrors.tuna.tsinghua.edu.cn/pypi/web/simple"
-ARG COMPILE_CUSTOM_KERNELS=1
 ARG MOONCAKE_TAG=v0.3.7.post2
 ARG SOC_VERSION="ascend910_9391"
 
 COPY . /vllm-workspace/vllm-ascend/
 # Define environments
 ENV DEBIAN_FRONTEND=noninteractive
-ENV COMPILE_CUSTOM_KERNELS=${COMPILE_CUSTOM_KERNELS}
 ENV SOC_VERSION=$SOC_VERSION
 
 RUN pip config set global.index-url ${PIP_INDEX_URL}