Revert "Cache node IDs in a hash table for faster lookups."

This reverts commit c2319e90b16962fe899bc60abc8af0e2542bb176.

As a general principle, I do not believe it is worthwhile to cache
nodes. Sure, it brings lookup time down from O(log n) to O(1), but
considering that the scalability target of tinc is around 1000 nodes
and log2(1000) is 10, that looks like premature optimization; tree
lookups should already be very fast. Therefore, I believe it makes sense
to remove the cache as a code cleanup initiative.