This reverts commit 15a8435.

Thomas Risch reported, that he is using two applications using Flash
technology.

1.  http://www.mapequation.org/apps/NetworkNavigator.html
2.  GGA system: http://gga.molgen.mpg.de

So add the Adobe Flash Player back.

Update the old version from January 2012 to the current one, which
probably contains a bunch of bug and security fixes. Unfortunately, I
didn’t find a change-log for the 11.2 version, and just the “normal” one
[1].

[1] https://helpx.adobe.com/flash-player/flash-player-releasenotes.html