fix(cfl): remove invalid -Z sanitizer=undefined from UBSan build

Rust's -Z sanitizer flag does not accept 'undefined' — valid values are
address, memory, thread, leak, etc.  UBSan is a C/C++ concept with no
direct Rust equivalent.  For the CFL UBSan job, build with
--sanitizer none so the binary has no ASan instrumentation and
bad_build_check passes, without attempting to set a non-existent
Rust sanitizer.

https://claude.ai/code/session_01EJLZKDtcF1RGdKx6TcUTQG

Author: claude

.clusterfuzzlite/build.sh

@@ -7,10 +7,9 @@ SANITIZER="${SANITIZER:-address}"
 if [ "$SANITIZER" = "address" ]; then
   cargo +nightly fuzz build --release
 elif [ "$SANITIZER" = "undefined" ]; then
-  # cargo fuzz defaults to ASan; for UBSan we suppress that and inject UBSan
-  # via RUSTFLAGS so the bad_build_check does not flag a sanitizer mismatch.
-  RUSTFLAGS="-Z sanitizer=undefined" \
-    cargo +nightly fuzz build --sanitizer none --release
+  # Rust has no UBSan equivalent via -Z sanitizer.
+  # Build without any sanitizer so bad_build_check sees no ASan instrumentation.
+  cargo +nightly fuzz build --sanitizer none --release
 else
   cargo +nightly fuzz build --release
 fi