Remove mapRequestCount from wallet

(I'm not 100% sure about this -- maybe these statistics are useful to keep track of for other reasons, just throwing this out there as a possibility)

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.