Removed the now-unused runner_doiact_fft.c file.

examples/EAGLE_12/eagle_12.yml

src/Makefile.am

src/engine.c

src/mesh_gravity.c

src/runner.c

src/runner_doiact_fft.c

src/runner_doiact_fft.h

src/scheduler.c

src/task.c

src/task.h