Update cores/arduino/zephyrCommon.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: soburi

cores/arduino/zephyrCommon.cpp

@@ -359,7 +359,6 @@ void tone_expiry_cb(struct k_timer *timer) {
 
 void tone(pin_size_t pinNumber, unsigned int frequency,
           unsigned long duration) {
-  size_t timerIndex = -1;
   struct k_timer *timer;
   k_timeout_t timeout;