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/BlockV.cpp

@@ -14,8 +14,6 @@ blockclass::blockclass()
 	rect.w = wp;
 	rect.h = hp;
 
-	prompt = "";
-	script = "";
 	r = 0;
 	g = 0;
 	b = 0;