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";