Get rid of checkpoints

[Johannes Köster]

Checkpoints are not needed here anymore, because the report flag now can take a directory and add all contained files into the report, see here.

Hence, we need to replace the boostrap jobs with a single job that iterates over all significant genes and writes the boostrap plots into a directory. That can then be annotated for the report as follows:

report("results/plots/bootstrap", patterns=["{transcript}.bootstrap.pdf"], category=..., caption=...)