Remove unnecessary use of shared_from_this()

If the request is cancelled, the response callback won't be called, and
~Source ensures that the request is cancelled. So it's safe to bind
`this` in the lambda.

src/mbgl/map/source.cpp

@@ -144,11 +144,9 @@ void Source::load(const std::string& accessToken) {
         return;
     }
 
-    util::ptr<Source> source = shared_from_this();
-
     const std::string url = util::mapbox::normalizeSourceURL(info.url, accessToken);
-    req = Environment::Get().request({ Resource::Kind::JSON, url }, [source](const Response &res) {
-        source->clearRequest();
+    req = Environment::Get().request({ Resource::Kind::JSON, url }, [this](const Response &res) {
+        req = nullptr;
 
         if (res.status != Response::Successful) {
             Log::Warning(Event::General, "Failed to load source TileJSON: %s", res.message.c_str());
@@ -163,10 +161,10 @@ void Source::load(const std::string& accessToken) {
             return;
         }
 
-        source->info.parseTileJSONProperties(d);
-        source->loaded = true;
+        info.parseTileJSONProperties(d);
+        loaded = true;
 
-        source->emitSourceLoaded();
+        emitSourceLoaded();
     });
 }
 
@@ -509,10 +507,6 @@ void Source::onLowMemory() {
     cache.clear();
 }
 
-void Source::clearRequest() {
-    req = nullptr;
-}
-
 void Source::setObserver(Observer* observer) {
     observer_ = observer;
 }

src/mbgl/map/source.hpp

@@ -51,7 +51,7 @@ public:
     std::string tileURL(const TileID& id, float pixelRatio) const;
 };
 
-class Source : public std::enable_shared_from_this<Source>, private util::noncopyable {
+class Source : private util::noncopyable {
 public:
     class Observer {
     public:
@@ -88,7 +88,6 @@ public:
 
     void setCacheSize(size_t);
     void onLowMemory();
-    void clearRequest();
 
     void setObserver(Observer* observer);