Bug#1126180: Remove Recommends: dvipng

[Youssef Adly]

Hi,

I can confirm that `dvipng` is not being used by xournalpp anymore.

A PR to remove it from xournalpp build docs, and Fedora package recommendations
has been merged.

PR: https://github.com/xournalpp/xournalpp/pull/7735


Thanks,
J. Adly