test(panic): remove WDT both CPU test

Test never worked on S3/P4 and was flakey on ESP32. Hard to design a reliable test
case that triggers both WDT at the exact same time.

tools/test_apps/system/panic/main/include/test_panic.h

@@ -32,7 +32,6 @@ void test_panic_extram_stack(void);
 
 #if !CONFIG_FREERTOS_UNICORE
 void test_task_wdt_cpu1(void);
-void test_task_wdt_both_cpus(void);
 #endif
 
 void test_storeprohibited(void);