- Official Post
I bet Steam OS has the capability to re-install the boot loader (BIOS MBR in your case). Then you don't need an unwanted extra Linux.
The point is that sometimes (like after major Win 10 updates) you have to re-install the boot loader. Do a research whether Steam OS has such capability, just to be sure. The boot loader will find all installed OS'es, including LE.
I'm not sure whether Win 10 likes the shrinking. If you have to re-install all OS's, do Win 10 first, then Steam OS (if it can re-install the boot loader), then LE.