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.cpp

@@ -76,8 +76,8 @@ namespace Catch {
 
     // This is the overload that takes a string and infers the Equals matcher from it
     // The more general overload, that takes any string matcher, is in catch_capture_matchers.cpp
-    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str, StringRef matcherString  ) {
-        handleExceptionMatchExpr( handler, Matchers::Equals( str ), matcherString );
+    void handleExceptionMatchExpr( AssertionHandler& handler, std::string const& str ) {
+        handleExceptionMatchExpr( handler, Matchers::Equals( str ) );
     }
 
 } // namespace Catch