Remove engine_marktasks() entirely

This is a relic of a long-gone past where marktasks and unskip needed to be different. Now it just makes use duplicate entire code blocks for no good reasons. engine_unskip() is superior as it only loops over the tree sections where actual work could happen, so that's the version we should keep.

This version keeps marktasks as a way to verify unskip's job. Once we are happy they are identical (as they should) we can remove that check.

Actual changes to keep past these tests:

• Better names for the various BH ghosts between the task names and the cell-carried task pointers.
• More uniform activation (for readability) of the gravity drift related to MPI between unskip and marktasks.
• Remove the activation of some gas drifts when neighbouring a BH cell where the BH is not active.
• Fix a mismatch where marktasks was activating unnecessary star pairs.
• Fix a mismatch where marktasks was activating unnecessary BH ghosts.
• Fix a mismatch where markatask was not activating star drifts in the case of stars doing only gravity.
• Fix a mismatch where marktasks was not activating RT_in tasks.