Remove deprecated method decayShortTermImportance and associated references

The method **decayShortTermImportance** was already marked as deprecated in **AtomSpace.h** [here](https://github.com/opencog/opencog/blob/b47f21d8d4ad4b562877b330b742befa2b321d58/opencog/atomspace/AtomSpace.h#L965). In the future, embodiment should use the attention allocation system to update short-term importance instead. This code no longer needs to be retained in the ImportanceIndex class.

This is to prepare for improvements to the ImportanceIndex class. Since there is an implementation of Attention Allocation, it doesn't make sense to have the redundant overhead of maintaining this deprecated method.