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_avr.go

@@ -4,7 +4,6 @@ package runtime
 
 import (
 	"device/avr"
-	"machine"
 	"unsafe"
 )
 
@@ -59,14 +58,6 @@ func init() {
 	initUART()
 }
 
-func initUART() {
-	machine.UART0.Configure(machine.UARTConfig{})
-}
-
-func putchar(c byte) {
-	machine.UART0.WriteByte(c)
-}
-
 const asyncScheduler = false
 
 const tickNanos = 1024 * 16384 // roughly 16ms in nanoseconds