refactor: remove unused methods {CDBIterator,CCoinsViewDBCursor}::GetValueSize()

These methods haven't been used since the chainstate db cache has been
switched from per-tx to per-txout model years ago (PR #10195, commit
d342424301).

src/coins.h

@@ -142,7 +142,6 @@ public:
 
     virtual bool GetKey(COutPoint &key) const = 0;
     virtual bool GetValue(Coin &coin) const = 0;
-    virtual unsigned int GetValueSize() const = 0;
 
     virtual bool Valid() const = 0;
     virtual void Next() = 0;

src/dbwrapper.h

@@ -166,11 +166,6 @@ public:
         }
         return true;
     }
-
-    unsigned int GetValueSize() {
-        return piter->value().size();
-    }
-
 };
 
 class CDBWrapper

src/txdb.cpp

@@ -211,7 +211,6 @@ public:
 
     bool GetKey(COutPoint &key) const override;
     bool GetValue(Coin &coin) const override;
-    unsigned int GetValueSize() const override;
 
     bool Valid() const override;
     void Next() override;
@@ -257,11 +256,6 @@ bool CCoinsViewDBCursor::GetValue(Coin &coin) const
     return pcursor->GetValue(coin);
 }
 
-unsigned int CCoinsViewDBCursor::GetValueSize() const
-{
-    return pcursor->GetValueSize();
-}
-
 bool CCoinsViewDBCursor::Valid() const
 {
     return keyTmp.first == DB_COIN;