attiny: remove dummy UART

I think it's better not to provide a UART0 global at all than one that
does nothing.

src/runtime/runtime_attiny.go

@@ -6,6 +6,13 @@ import (
 	"device/avr"
 )
 
+func initUART() {
+}
+
+func putchar(c byte) {
+	// UART is not supported.
+}
+
 func sleepWDT(period uint8) {
 	// TODO: use the watchdog timer instead of a busy loop.
 	for i := 0x45; i != 0; i-- {