Removed redundant duplicate type definition.

Author: dok-net

cores/esp8266/core_esp8266_main.cpp

@@ -162,8 +162,6 @@ extern "C" void __esp_delay(unsigned long ms) {
 
 extern "C" void esp_delay(unsigned long ms) __attribute__((weak, alias("__esp_delay")));
 
-using IsBlockedCB = std::function<bool()>;
-
 void esp_delay(const uint32_t timeout_ms, const IsBlockedCB& blocked, const uint32_t intvl_ms) {
     const auto start = millis();
     decltype(millis()) expired;