Fix extended bins in guide_coloursteps()

[thomasp85]

Fix #4019

This PR fixes the issue with extended terminal bins in guide_coloursteps() by removing breaks if they coincide

[yutannihilation]

Looks good.

devtools::load_all("~/repo/ggplot2")
#> ℹ Loading ggplot2

df <- data.frame(x = 0:4)

ggplot(df, aes(x, 0)) +
  geom_point(aes(fill = x), pch = 21, size = 80) +
  scale_fill_steps(low = "white", high = "red", limits = c(1, 3), oob = scales::censor)

^{Created on 2021-04-16 by the reprex package (v2.0.0)}