avr: remove I2C stubs from attiny support

These stubs don't really belong there: attiny currently doesn't directly
support I2C at all (although it has hardware to support a software
implementation).

src/machine/i2c.go

@@ -1,4 +1,4 @@
-// +build avr nrf sam stm32,!stm32l0 fe310 k210
+// +build atmega nrf sam stm32,!stm32l0 fe310 k210
 
 package machine
 

src/machine/machine_atmega.go

@@ -9,6 +9,13 @@ import (
 	"unsafe"
 )
 
+// I2C on AVR.
+type I2C struct {
+}
+
+// I2C0 is the only I2C interface on most AVRs.
+var I2C0 = I2C{}
+
 // I2CConfig is used to store config info for I2C.
 type I2CConfig struct {
 	Frequency uint32

src/machine/machine_attiny.go

@@ -1,9 +0,0 @@
-// +build avr,attiny
-
-package machine
-
-// Tx is a dummy implementation. I2C has not been implemented for ATtiny
-// devices.
-func (i2c I2C) Tx(addr uint16, w, r []byte) error {
-	return nil
-}

src/machine/machine_avr.go

@@ -141,10 +141,3 @@ func (a ADC) Get() uint16 {
 
 	return uint16(avr.ADCL.Get()) | uint16(avr.ADCH.Get())<<8
 }
-
-// I2C on AVR.
-type I2C struct {
-}
-
-// I2C0 is the only I2C interface on most AVRs.
-var I2C0 = I2C{}