How do I get glass walls without borders?

Aside from texture pack possibilities, there are two different types of glass in the stock game -- Glass, and Glass Pane. One looks like a window with an outline, the other looks more like a wall of glass.

You can create 16x Glass Pane using 6x Glass and a Crafting Table.

I assume you mean something like this:

AFAIK, you can only do this with a mod called Optifine:

1. Close Minecraft and the Launcher.
2. Go to the download page and download Optifine HD Ultra for the MC version you are using.
3. Open the .jar file you just downloaded (using java!) and click install.

When it is finished, close that and open Minecraft. There should be a profile called "Optifine". Select it and click play. All of your glass blocks will be changed to look like in the first screenshot.

I made this texture pack which works without Optifine, you could try it https://www.curseforge.com/minecraft/texture-packs/clear-glass-without-optifine