doc: Remove misleading checkpoints comment in CMainParams #14149

This removes the checkpoints comment because it is misleading for two reasons:

• It shows up in the doxygen documentation of CMainParams https://dev.visucore.com/bitcoin/doxygen/class_c_main_params.html
• The comment refers to "strange transactions" in a block, which are not specified further. Transactions in blocks are always consensus-valid or rejected as consensus-invalid.

Also sort the includes with clang-format, as the file is touched anyway.

<!--e57a25ab6845829454e8d69fc972939a-->Note to reviewers: This pull request conflicts with the following ones:

• #13311 (Don't edit Chainparams after initialization by jtimon)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

utACK checkpoints no longer consider timestamps and the strange transactions comment was always nonsensical

utACK fab5267.

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