Remove icon templates because Jolla released official templates

Hi,

Jolla has released official icon templates and guidelines about the icon look. The icon size is the same as before (86x86) but there are more accurate guides how the content inside the icon should look like.

The official icons are available at https://sailfishos.org/apps_icons.html
The icons are CC-BY licensed, but according to their readme.txt, no attribution is needed, making their icons kind of public domain.

I also found Morpog's icon templates at https://github.com/Morpog/jolla-icon-shapes
However, the license is CC-BY, not public domain.

Instead of having plenty of different icon templates (and the need to update them), I think it is better to have only few. So, I removed the icon templates from helloworld-pro. Another option would have been to update them to match Jolla's icon look guidelines.

I also removed all comments about icon templates in README.txt. If you want, you can perhaps add a link to Jolla's official icons?

And sorry if my requests to add and then remove the icon templates has unnecessarily wasted your time..

Cheers,
Kari