Remove Qt < 5.2 conditionals and special code

[Jürgen Spitzmüller]

According to the README, we require Qt 5.2 or higher for 2.4.

There is still some code for Qt < 5.2 in the source. Particularly a
self-baked "Don't ask me again" warning dialog.

OK to remove that (see patch)?

-- 
Jürgen
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