coinselection: Remove COutput operators == and !=

These operators are used only by the tests in std::mismatch. As
std::mismatch can take a binary predicate, we can use a lambda that
achieves the same instead.

src/wallet/test/coinselector_tests.cpp

@@ -113,7 +113,10 @@ static bool EquivalentResult(const SelectionResult& a, const SelectionResult& b)
 /** Check if this selection is equal to another one. Equal means same inputs (i.e same value and prevout) */
 static bool EqualResult(const SelectionResult& a, const SelectionResult& b)
 {
-    std::pair<CoinSet::iterator, CoinSet::iterator> ret = std::mismatch(a.GetInputSet().begin(), a.GetInputSet().end(), b.GetInputSet().begin());
+    std::pair<CoinSet::iterator, CoinSet::iterator> ret = std::mismatch(a.GetInputSet().begin(), a.GetInputSet().end(), b.GetInputSet().begin(),
+        [](const COutput& a, const COutput& b) {
+            return a.outpoint == b.outpoint;
+        });
     return ret.first == a.GetInputSet().end() && ret.second == b.GetInputSet().end();
 }