drm/nouveau/fifo: rip out cevent, never used

drivers/gpu/drm/nouveau/include/nvkm/engine/fifo.h

drivers/gpu/drm/nouveau/nvkm/engine/fifo/base.c

drivers/gpu/drm/nouveau/nvkm/engine/fifo/chan.c

drivers/gpu/drm/nouveau/nvkm/engine/fifo/priv.h