LibC+Everywhere: Remove open_with_path_length() in favor of open()

This API was a mostly gratuitous deviation from POSIX that gave up some
portability in exchange for avoiding the occasional strlen().

I don't think that was actually achieving anything valuable, so let's
just chill out and have the same open() API as everyone else. :^)

Userland/Shell/AST.cpp

@@ -33,6 +33,7 @@
 #include <AK/URL.h>
 #include <LibCore/EventLoop.h>
 #include <LibCore/File.h>
+#include <fcntl.h>
 #include <signal.h>
 
 //#define EXECUTE_DEBUG