You want to remove a legend title.

Add guides(fill=guide_legend(title=NULL)) to remove the title from a legend, as in Figure 10-12 (for other aesthetics, replace fill with the name of the aesthetic, such as colour or size):

ggplot(PlantGrowth, aes(x=group, y=weight, fill=group)) + geom_boxplot() +
    guides(fill=guide_legend(title=NULL))

Figure 10-12. Box plot with no legend title

It is also possible to control the legend title when specifying the scale. This has the same effect as the preceding code:

scale_fill_hue(guide = guide_legend(title=NULL))