Revert "Actually use mapAlreadyAskedFor."

This reverts commit 643160f6e7.

Turns out this commit was useless after a more careful reading of
CNode::AskFor

src/main.cpp

@@ -3994,7 +3994,6 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
                     pto->PushMessage("getdata", vGetData);
                     vGetData.clear();
                 }
-                mapAlreadyAskedFor[inv] = nNow;
             }
             pto->mapAskFor.erase(pto->mapAskFor.begin());
         }