Update arduino_i2c.py

I found that I was getting a 9 sent to the Arduino, Uno in my case.
By removing the gpio_register from i2c.writing_bytes(address, gpio_register, value) I found this stopped the 9 coming through.

I'm fairly new at python and programming in general however I'm unsure as to why you need the iodir_register and gpio_register there unless you are using an I2C GPIO expander.