disable debugging groups in non-debug builds

Use a macro to completely remove all instantiations of mbgl::gl::debugging::group so that we aren't executing the code to generate the arguments to the object instantiation

src/mbgl/gl/debugging.cpp

@@ -169,7 +169,6 @@ void enable() {
     MBGL_CHECK_ERROR(DebugMessageCallback(debugCallback, nullptr));
 }
 
-#if defined(DEBUG)
 group::group(const std::string& str) {
     if (PushDebugGroup) {
         PushDebugGroup(GL_DEBUG_SOURCE_APPLICATION, 0, GLsizei(str.size()), str.c_str());
@@ -185,10 +184,6 @@ group::~group() {
         PopGroupMarkerEXT();
     }
 }
-#else
-group::group(const std::string&) {}
-group::~group() = default;
-#endif
 
 }
 }