Qt: remove unneeded constructors.

Qt: remove unneeded constructors.

There is no need to have an empty private constructor if an explicit
one is already present. The former private constructor didn't initialize
the private members causing many CIDs from coverity.

Change-Id: I8ba98d2b9fc32a867fdcf3421df7946b90bd7b09
Reviewed-on: https://code.wireshark.org/review/16311
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>