Remove mapRequestCount from wallet

Maprequestcount in the wallet is used to keep track of the number of requests
that a certain block/transaction has had.

However, this does not affect the behavior in any way, and is not
reported over RPC in any way. It is only used for some obscure stats in the GUI
which I think would be slightly useful for people solo-mining using the GUI.

This also makes the "Inventory" signal no longer necessary.