wallet: remove unused COutput::ToString

No callers. It was used by `COutput::print()`, which was removed in
3802224110 ("Remove all other print() methods").

src/wallet/coinselection.cpp

@@ -1027,11 +1027,6 @@ bool SelectionResult::operator<(SelectionResult other) const
     return *m_waste < *other.m_waste || (*m_waste == *other.m_waste && m_selected_inputs.size() > other.m_selected_inputs.size());
 }
 
-std::string COutput::ToString() const
-{
-    return strprintf("COutput(%s, %d, %d) [%s]", outpoint.hash.ToString(), outpoint.n, depth, FormatMoney(txout.nValue));
-}
-
 std::string GetAlgorithmName(const SelectionAlgorithm algo)
 {
     switch (algo)

src/wallet/coinselection.h

@@ -98,8 +98,6 @@ public:
         effective_value = txout.nValue - fee.value();
     }
 
-    std::string ToString() const;
-
     bool operator<(const COutput& rhs) const
     {
         return outpoint < rhs.outpoint;