boards: Remove support for the Jiminy-Mega256RFR2

- The Jiminy-Mega256RFR2 is seemingly unmaintained, as no one has responded
  on requests to test changes on it. This lack of support has been blocking
  various PRs in the last few months
- No one seems to use it
- Boards that are added to RIOT should match at least one of the following
    - It is actively maintained
    - It has a significant user base
    - It is an off-the-shelf product, so it might be easily obtained for testing
==> The Jiminy is matching none of the above

Author: maribu