Remove deal.II versioning from README files

[jppelteret]

Some of the codes mention the version requirement for deal.II. This is no longer necessary, and should be removed.

[bangerth]

I can only find this in the preCISE example:

* `deal.II`, version `9.2` or greater. Older versions might work as well, but have not been tested.

This seems reasonable to leave in. Do you disagree, @jppelteret ?