Remove unnecessary string initializations from constructors

std::string is one of those special types that has a constructor that
just initializes itself to a blank state automatically. This means all
`std::string`s are by default already `""`, so there's no need to set
them. And in fact, cppcheck throws out warnings about performance due to
initializing `std::string`s this way.

desktop_version/src/editor.cpp

@@ -33,7 +33,6 @@ edlevelclass::edlevelclass()
 {
     tileset=0;
     tilecol=0;
-    roomname="";
     warpdir=0;
     platx1=0;
     platy1=0;