Get rid of zero-length poll case now that timer pool exists

src/core/lib/iomgr/iomgr.c

src/core/lib/iomgr/timer.h

src/core/lib/iomgr/timer_generic.c

src/core/lib/iomgr/timer_manager.c

src/core/lib/iomgr/timer_uv.c

test/core/iomgr/tcp_client_posix_test.c

test/core/iomgr/tcp_client_uv_test.c

test/core/iomgr/timer_list_test.c