Remove one regression line but not the other using geom_smooth in ggplot2

Hi, and welcome.

Could you post a reprex: FAQ: What's a reproducible example (`reprex`) and how do I do one?

Although the code is in your question, there's no data. You can reproduce it with

df. <- dput(cortisoldiffdata)

otherwise, folk will have to look for a similar dataset.

Also, it would be preferable to strip the reprex to the essential elements

RAMPcortisol <- ggplot(data = cortisoldiffdata, mapping = aes(x = cortisoldiff, y = correctramp, fill = season, colour = season)) +
geom_point(size = 1.75, alpha = 3/4) +
geom_smooth(method='lm' ...