Remove libqgtk3.so and other platformthemes that may lead to crashes

[probonopd]

Closes #150

[probonopd]

Getting

-- Running input plugin: qt -- 

Running process: /tmp/AppImageUpdate-build-F1P5tL/linuxdeploy-plugin-qt-x86_64.AppImage --appdir AppImageUpdate.AppDir

munmap_chunk(): invalid pointer

resources/build-appimages.sh: line 78:  8203 Aborted                 (core dumped) ./linuxdeploy-"$ARCH".AppImage --appdir 

Possibly caused by linuxdeploy/linuxdeploy#143.

[TheAssassin]

There's no reason to believe this is a good idea, other than some random experiments in #150. Mind to explain why how these are causing a crash? I think it was you who suggested to bundle these platform themes in the first place, so it would be appreciated if you could explain why they now fail, and whether there's changes needed in tools like linuxdeploy-plugin-qt (if it's a global issue, others could benefit from a solution as well).

[TheAssassin]

See above.

[probonopd]

While I don't fully understand yet why they fail for some users, they do seem to be problematic, as removing them fixes the problems various users have reported.

And I was advocating for deploying libqgtk2* (without its dependencies - like Scribus is doing), not libqgtk3*. Actually it would be a good test if the affected users could try the Scribus AppImage and see if they experience the same crash with it.

[TheAssassin]

Possibly related: #151 (comment).

[probonopd]

Two years later, I was writing the exact same PR again when I found this.

We are getting lots and lots of error reports since years of AppImageUpdate crashing due to Gtk errors, and I am now sick and tired of it. Let's just remove all and everything related to Gtk and call it a day.

Unfortunately it seems that the LinuxDeploy plugin for Qt is what pulls this Gtk stuff in and there seems to be no obvious way to disable it...

[probonopd]

Please retry with https://github.com/AppImage/AppImageUpdate/releases/tag/continuous.