Remove Qt < 5.2 conditionals and special code

[Pavel Sanda]

On Thu, Aug 17, 2023 at 02:40:44PM +0200, Jürgen Spitzmüller wrote:
> But this would mean, in consequence, that if we implement something
> within the 2.4 cycle, we would have to make sure it still compiles with
> Qt < 5.2.
> 
> I got to this while pondering about the URL link warning dialog.
> Implementing this dialog (the way I proposed it) would mean we'd need
> to roll another self-baked dialog only for the sake of keeping it
> compilable with Qt < 5.2.

I don't think there is a value in keeping LyX compilable with Qt < 5.2,
because 5.2 was released 10 years ago. 
Even closed/commerical LTS variants of current distros do not show support >10y.

So +1 for removal from my side.

Pavel