fix: remove CUB_WRAPPED_NAMESPACE and _CCCL_COMPILER_GCC from build flags

CUB_WRAPPED_NAMESPACE=cccl_preload wraps cub into cccl_preload::cub
but cccl_allocator_preload.cu uses bare cub:: — compilation fails.
_CCCL_COMPILER_GCC=1 conflicts with CCCL auto-detection (redefined warning).

Drop both. CUB_DISABLE_NAMESPACE_MAGIC alone is sufficient.

qwen3_6_scripts/cccl_preload/build_cccl_preload.sh

@@ -72,11 +72,9 @@ COMMON_FLAGS=(
     # Suppress CCCL warnings that don't affect correctness
     -Wno-unused-function
     -Wno-unknown-pragmas
-    # CUB needs these for non-NVCC compilers
-    -D_CCCL_COMPILER_GCC=1
+    # CUB needs this for non-NVCC compilers
     -D__CUDA_ARCH_LIST__=700
     -DCUB_DISABLE_NAMESPACE_MAGIC
-    -DCUB_WRAPPED_NAMESPACE=cccl_preload
 )
 
 if [[ "${CXX}" == *clang++* ]]; then