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

include/mbgl/ios/MGLMapView+MGLCustomStyleLayerAdditions.h

@@ -20,6 +20,8 @@ typedef void (^MGLCustomStyleLayerCompletionHandler)(void);
 
 - (void)insertCustomStyleLayerWithIdentifier:(NSString *)identifier preparationHandler:(MGLCustomStyleLayerPreparationHandler)preparation drawingHandler:(MGLCustomStyleLayerDrawingHandler)drawing completionHandler:(MGLCustomStyleLayerCompletionHandler)completion belowStyleLayerWithIdentifier:(nullable NSString *)otherIdentifier;
 
+- (void)removeCustomStyleLayerWithIdentifier:(NSString *)identifier;
+
 - (void)setCustomStyleLayersNeedDisplay;
 
 @end

include/mbgl/map/map.hpp

@@ -174,6 +174,7 @@ public:
                         CustomLayerDeinitializeFunction,
                         void* context,
                         const char* before = nullptr);
+    void removeCustomLayer(const std::string& id);
 
     // Memory
     void setSourceTileCacheSize(size_t);