TrackerContext: Remove misleading instance static method.

TrackerContext is not used as singleton, so just remove this misleading
method

include/internal/catch_test_case_tracker.cpp

@@ -32,11 +32,6 @@ namespace TestCaseTracking {
     ITracker::~ITracker() = default;
 
 
-    TrackerContext& TrackerContext::instance() {
-        static TrackerContext s_instance;
-        return s_instance;
-    }
-
     ITracker& TrackerContext::startRun() {
         m_rootTracker = std::make_shared<SectionTracker>( NameAndLocation( "{root}", CATCH_INTERNAL_LINEINFO ), *this, nullptr );
         m_currentTracker = nullptr;

include/internal/catch_test_case_tracker.h

@@ -71,8 +71,6 @@ namespace TestCaseTracking {
 
     public:
 
-        static TrackerContext& instance();
-
         ITracker& startRun();
         void endRun();