greybus: Revert "interface: Fetch and expose version of interface's firmware"

This reverts commit b957ade7b3e4ab8c149c53346dbf02e977b7f3a7.

The interface version is now managed as part of the firmware-management
protocol. This operation is already removed from the greybus
specifications.

Drop interface version support from greybus.

Tested with gbsim (sysfs file not available after this patch).

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Acked-by: Sandeep Patil <sspatil@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>