Clean up C++ warnings (#6213)

src/common/io.h

@@ -32,7 +32,7 @@ class PeekableInStream : public dmlc::Stream {
   size_t Read(void* dptr, size_t size) override;
   virtual size_t PeekRead(void* dptr, size_t size);
 
-  void Write(const void* dptr, size_t size) override {
+  void Write(const void*, size_t) override {
     LOG(FATAL) << "Not implemented";
   }
 
@@ -60,7 +60,7 @@ class FixedSizeStream : public PeekableInStream {
   size_t Tell() const { return pointer_; }
   void Seek(size_t pos);
 
-  void Write(const void* dptr, size_t size) override {
+  void Write(const void*, size_t) override {
     LOG(FATAL) << "Not implemented";
   }