[XPU][CI] Drop the USTC PyPI mirror from the XPU image build (#7808)

Signed-off-by: Joshna Medisetty <joshna.medisetty@intel.com>

docker/Dockerfile.xpu

@@ -40,7 +40,6 @@ ENV VLLM_OMNI_TARGET_DEVICE=xpu
 # Keep the upstream oneCCL packages paired with the XPU torch runtime.
 RUN --mount=type=cache,target=/root/.cache/uv \
     uv pip install --no-build-isolation \
-    --extra-index-url https://mirrors.ustc.edu.cn/pypi/simple/ \
     ".[dev]" && \
     uv pip uninstall triton && \
     uv pip install triton-xpu==3.7.2 --reinstall