Remove IStreamingReporter::isMulti query

This was previously used to avoid `dynamic_cast` inside our code,
when we were creating more than one reporter, or a reporter
together with listeners. However, since then the offending code
was refactored to be smarter instead, and this query member function
is no longer needed nor used.

src/catch2/catch_interfaces_reporter.cpp

@@ -112,7 +112,6 @@ namespace Catch {
     TestRunStats::~TestRunStats() = default;
 
     void IStreamingReporter::fatalErrorEncountered( StringRef ) {}
-    bool IStreamingReporter::isMulti() const { return false; }
 
     void IStreamingReporter::listReporters(std::vector<ReporterDescription> const& descriptions, Config const& config) {
         Catch::cout() << "Available reporters:\n";

src/catch2/catch_interfaces_reporter.h

@@ -241,8 +241,6 @@ namespace Catch {
         // Default empty implementation provided
         virtual void fatalErrorEncountered( StringRef name );
 
-        virtual bool isMulti() const;
-
         // Listing support
         virtual void listReporters(std::vector<ReporterDescription> const& descriptions, Config const& config);
         virtual void listTests(std::vector<TestCaseHandle> const& tests, Config const& config);

src/catch2/reporters/catch_reporter_listening.cpp

@@ -150,10 +150,6 @@ namespace Catch {
         m_reporter->skipTest( testInfo );
     }
 
-    bool ListeningReporter::isMulti() const {
-        return true;
-    }
-
     void ListeningReporter::listReporters(std::vector<ReporterDescription> const& descriptions, Config const& config) {
         for (auto const& listener : m_listeners) {
             listener->listReporters(descriptions, config);

src/catch2/reporters/catch_reporter_listening.h

@@ -50,7 +50,6 @@ namespace Catch {
         void testRunEnded( TestRunStats const& testRunStats ) override;
 
         void skipTest( TestCaseInfo const& testInfo ) override;
-        bool isMulti() const override;
 
         void listReporters(std::vector<ReporterDescription> const& descriptions, Config const& config) override;
         void listTests(std::vector<TestCaseHandle> const& tests, Config const& config) override;