Drop return value from Serial::write() (#21567)

Marlin/src/HAL/AVR/MarlinSerial.cpp

 }
 
 template<typename Cfg>
-size_t MarlinSerial<Cfg>::write(const uint8_t c) {
+void MarlinSerial<Cfg>::write(const uint8_t c) {
   if (Cfg::TX_SIZE == 0) {
 
     _written = true;
       // location". This makes sure flush() won't return until the bytes
       // actually got written
       B_TXC = 1;
-      return 1;
+      return;
     }
 
     const uint8_t i = (tx_buffer.head + 1) & (Cfg::TX_SIZE - 1);
     // Enable TX ISR - Non atomic, but it will eventually enable TX ISR
     B_UDRIE = 1;
   }
-  return 1;
 }
 
 template<typename Cfg>

Marlin/src/HAL/AVR/MarlinSerial.h

     static int read();
     static void flush();
     static ring_buffer_pos_t available();
-    static size_t write(const uint8_t c);
+    static void write(const uint8_t c);
     static void flushTX();
     #if HAS_DGUS_LCD
       static ring_buffer_pos_t get_tx_buffer_free();

Marlin/src/core/serial_base.h

 
   // Static dispatch methods below:
   // The most important method here is where it all ends to:
-  size_t write(uint8_t c)           { return SerialChild->write(c); }
+  void write(uint8_t c)             { SerialChild->write(c); }
 
   // Called when the parser finished processing an instruction, usually build to nothing
   void msgDone() const              { SerialChild->msgDone(); }