java_gen: use auto-generated reader/writer code for OFBsnVportQInQ

Reviewer: @shudong @rlane

This commit removes the unimplemented manual stub methods for reading
and writing OFBsnVportQInq from ChannelUtilsVer${version}. Instead,
it delegates the OP to the auto-generated reader and writer.

Caveat: It seems the c code does not correctly honor the format
defined in openflow_input at this time. Based on the input, the
auto-generated reader and writer start the vport with a type and
a length field:
    uint16_t type == 0;
    uint16_t length;  /* 32 */
This is inconsistent with the implementation in bigdb and also
what the switch seems to expect.