How to get ride of polygon borders using geom_sf in ggplot2

This question has been asked before in an old thread, but the accepted answer does not anymore in the current version of ggplot2. Here is a minimal example:

library(ggplot2)
library(rnaturalearth)
world = ne_countries(scale = "medium", returnclass = "sf")
ggplot(world) + 
  geom_sf(aes(fill = pop_est)) + 
  scale_fill_viridis_c(option = "plasma", trans = "sqrt")

My question is: how can I get rid of the annoying borders in each country?

Thank you!