remove method changed for scalebar

[schloerke]

Fixes: #538 Scalebar not removed properly in v2.0.0

library(leaflet)                                                    
                                                                    
leaf <- leaflet() %>%                                               
  addTiles() %>%                                                      
  addScaleBar()                                                       
                                                                    
# displays scalebar in top right                                    
leaf                                                                

                                                                    
# no scalebar displayed in top right                                
leaf %>% removeScaleBar()                                           

# displays scalebar in top right with blue polygons over switzerland
leaflet() %>%                                                       
  addTiles() %>%                                                      
  addScaleBar() %>%                                                   
  addScaleBar() %>% # triggers remove scalebar internally             
  addPolygons(data = gadmCHE)                                         
#> Loading required package: sp

PR task list:

• Update NEWS

[schloerke]

from https://leafletjs.com/reference-0.7.7.html#control-scale to https://leafletjs.com/reference-1.3.0.html#control-scale