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

src/interfaces/chain.h

@@ -155,6 +155,9 @@ public:
 
     //! Pool min fee.
     virtual CFeeRate mempoolMinFee() = 0;
+
+    //! Check if pruning is enabled.
+    virtual bool getPruneMode() = 0;
 };
 
 //! Interface to let node manage chain clients (wallets, or maybe tools for