There isn't. If you want a lightweight, fast text editor on the Mac, Sublime Text is pretty great!
You can also put RStudio in a mode that shows only the text editor, which might do what you want (you won't have "all of RStudio" staring at you). To do this, use the "Zoom Source" command from the palette or View menu (Ctrl + Shift + 1). Bam, RStudio is now just a text editor: