sandbox: Don't print a warning for CONFIG_I2C_COMPAT

Sandbox includes this code to provide build coverage. While we retain this
feature we should have sandbox build it. Sandbox does not in fact use the
I2C compatibility mode. Showing a warning for sandbox is just confusing,
since no conversion is expected.

Drop the warning for sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>