[ios, android] Add methods to remove a custom layer

src/mbgl/map/map.cpp

@@ -449,6 +449,10 @@ void Map::addCustomLayer(const std::string& id,
         before ? std::string(before) : mapbox::util::optional<std::string>());
 }
 
+void Map::removeCustomLayer(const std::string& id) {
+    context->invoke(&MapContext::removeLayer, id);
+}
+
 #pragma mark - Toggles
 
 void Map::setDebug(MapDebugOptions mode) {

src/mbgl/map/map_context.cpp

@@ -289,6 +289,12 @@ void MapContext::addLayer(std::unique_ptr<StyleLayer> layer, mapbox::util::optio
     asyncUpdate.send();
 }
 
+void MapContext::removeLayer(const std::string& id) {
+    style->removeLayer(id);
+    updateFlags |= Update::Classes;
+    asyncUpdate.send();
+}
+
 void MapContext::setSourceTileCacheSize(size_t size) {
     assert(util::ThreadContext::currentlyOn(util::ThreadType::Map));
     if (size != sourceCacheSize) {

src/mbgl/map/map_context.hpp

@@ -59,6 +59,7 @@ public:
     // Style API
     void addLayer(std::unique_ptr<StyleLayer>,
                   const mapbox::util::optional<std::string> before);
+    void removeLayer(const std::string& id);
 
     void setSourceTileCacheSize(size_t size);
     void onLowMemory();