clear_machine method actually prevents clearing the machine

[Christian-B]

Bug affects spin3 boards.

Script
import pyNN.spiNNaker as sim

sim.setup(timestep=1.0, n_boards_required=1)
sim.set_number_of_neurons_per_core(sim.IF_curr_exp, 100)

pop_1 = sim.Population(1, sim.IF_curr_exp(), label="pop_1")
input_pop = sim.Population(
1, sim.SpikeSourceArray(spike_times=[0]), label="input")
input_proj = sim.Projection(input_pop, pop_1, sim.OneToOneConnector(),
synapse_type=sim.StaticSynapse(weight=5, delay=1))
pop_1.record(["spikes", "v"])
sim.run(10)
sim.reset()
machine = sim.get_machine()
sim.run(30)
sim.end()

error:
Too much SDRAM has been allocated on chip 0, 0: 965726 of 951616

Caused by:
Detected Machine on 192.168.240.253 with 4 Chips, 65 cores and 5.0 links. Chips have sdram of between 951616 and 123456448 byte

deep cause:
self._data_writer.clear_machine()
called before
hard_reset

hard reset does
transceiver().stop_application

but the clear_machine has removed the tranceiver.

Fix
remove clear _machine method as hard reset does the same clear at the right time.