Remove uses of fPruneMode in wallet code

This commit does not change behavior.

src/interfaces/chain.cpp

@@ -228,6 +228,7 @@ public:
     {
         return ::mempool.GetMinFee(gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
     }
+    bool getPruneMode() override { return ::fPruneMode; }
 };
 
 } // namespace