I finally found a way to do this.
First, I installed dconf-editor:
sudo apt install dconf-editor
I then started this up and navigated to the following place:
I then scrolled down to the global-view-setting.
I set the Use default value to OFF, and then set the Custom value to True,
Once I turned off the preview for the Inbox, it was gone for all other message folders too:
View > Preview > Uncheck Show Message preview.
You can also use Ctrl+M to toggle this on and off.