Remove contents memset from editorclass constructor

It gets (re-) initialized in editorclass::reset() anyway, so I'm just
removing this for code clarity.

desktop_version/src/CustomLevels.cpp

@@ -54,9 +54,6 @@ edlevelclass::edlevelclass(void)
 
 editorclass::editorclass(void)
 {
-    //We create a blank map
-    SDL_memset(contents, 0, sizeof(contents));
-
     for (size_t i = 0; i < SDL_arraysize(vmult); i++)
     {
         vmult[i] = i * 40 * maxwidth;