now that we have a routine to clean up the packet-specific infos in the
usb conversation, there's no need to clean up usb_conv_info after
parsing the configuration descriptor

this wouldn't work anyway if one of the subfunctions throws an exception

Change-Id: Ifd13f320db1ec17c3b706778ce0f5dffa1349770
Reviewed-on: https://code.wireshark.org/review/3729
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>