Remove unused StringRef argument from MatchExpr

Apart from cleaning up the code, this change also improves the
compilation time of `UsageTests/Matchers.tests.cpp` by about 2%.

src/catch2/internal/catch_assertion_handler.hpp

@@ -64,7 +64,7 @@ namespace Catch {
         auto allowThrows() const -> bool;
     };
 
-    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str, StringRef matcherString );
+    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str );
 
 } // namespace Catch