Remove gpu_burn from ISO build — binary too large

gpu_burn requires CUDA toolkit (~4GB) to build and the resulting binary
would significantly inflate the ISO. Removed from vendor tool list and
smoketest. build-gpu-burn.sh dropped as well.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build.sh

@@ -99,7 +99,7 @@ cp "${DIST_DIR}/bee-audit-linux-amd64" "${OVERLAY_DIR}/usr/local/bin/audit"
 chmod +x "${OVERLAY_DIR}/usr/local/bin/audit"
 
 # --- vendor utilities (optional pre-fetched binaries) ---
-for tool in storcli64 sas2ircu sas3ircu mstflint gpu_burn; do
+for tool in storcli64 sas2ircu sas3ircu mstflint; do
     if [ -f "${VENDOR_DIR}/${tool}" ]; then
         cp "${VENDOR_DIR}/${tool}" "${OVERLAY_DIR}/usr/local/bin/${tool}"
         chmod +x "${OVERLAY_DIR}/usr/local/bin/${tool}" || true