cros_ec: Clean up multiple EC protocol support

Version 1 protocols (without command version) were already no longer
supported in cros_ec.c.  This removes some dead code from the
cros_ec_i2c driver.

Version 2 protcols (with command version) are now called
protocol_version=2, instead of cmd_version_is_supported=1.

A subsequent change will introduce protocol version 3 for SPI.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>