Support builds that completely exclude I2CDev support

[fhunleth]

This removes both the Elixir module and the NIF. There's no longer any unintended NIF builds or loads or anything extra.

To enable this, either set the :default_backend application environment key or force it by setting :include_i2c_dev. Most users probably want:

config :circuits_i2c, default_backend: MyI2CBackend

This fixes the issue reported at https://elixirforum.com/t/circuits-sim-in-a-release/72076/3.