Revert "Merge branch 'threadpool_task_plots' into 'master'

configure.ac

examples/main.c

examples/plot_threadpool.py

examples/process_plot_threadpool

src/engine.c

src/engine.h

src/gravity.c

src/queue.h

src/scheduler.c

src/space.c

src/statistics.c

src/swift.h

src/threadpool.c

src/threadpool.h

tests/testThreadpool.c