Remove jsdom v15 environment once jest v25 is out

[wtho]

Jest v25 does use JSDOM v15 by default, so we do not have to define this version in the preset anymore.

This issue is a reminder and can be solved by a simple PR once jest v25 is out.

[wtho]

There are also plans for v25 to set the default env to node.js instead of jsdom, so we might have to keep setting jsdom anyway, let's see!

[ahnpnl]

The plan to set default to jsdom is moved to v26, so we can just remove the dependency for now