Revert part of last commit unsuitable for older compilers (#3584)

Also initialise all the items in gResearchedTrackTypesA and
gResearchedTrackTypesB

src/rct2/S6Importer.cpp

@@ -50,6 +50,7 @@ public:
 S6Importer::S6Importer()
 {
     FixIssues = false;
+    memset(&_s6, 0, sizeof(_s6));
 }
 
 void S6Importer::LoadSavedGame(const utf8 * path)

src/rct2/S6Importer.h

@@ -41,6 +41,6 @@ public:
 
 private:
     const utf8 * _s6Path = nullptr;
-    rct_s6_data  _s6 = { 0 };
+    rct_s6_data  _s6;
     uint8        _gameVersion = 0;
 };