and adapts TM which used this function.

The functionality is removed as not plausible biologically, instead
we let a (weak) synapse decay even further, so it's removed.

this is adapted to the artificial destroyMinPermSynapses() removed,
instead we rely on decay synapse to eventually remove the unused syn.

obsolete removing least recently used segments if cell is full.
Instead, use plausible way for pruning segments with all synapses off.