remove unnecessary try_dissect_linux_usb_pseudo_header_ext() call

Change-Id: Iafe173d4625d7e57ee3bd9c2fe9a6e6176624dcc
Reviewed-on: https://code.wireshark.org/review/3899
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>