[node] Allow GC in spite of reference loops through the request method

The following produces a reference cycle:

var map = new mbgl.Map({
    request: function () { ... }
});

map has a reference to the request function, which has a reference to
its closure, which has a reference to map.

Storing a the map options in an internal field, rather than a Persistent,
allows V8 to collect the reference cycle when none of its members have
other references.

platform/node/src/node_request.cpp

@@ -1,5 +1,4 @@
 #include "node_request.hpp"
-#include "node_file_source.hpp"
 #include <mbgl/storage/response.hpp>
 
 #include <cmath>