Remove unused "mNormal" uniform to fix crash on GLES2 with shaders

[grorp]

Fixes #13841.

Clients with video_driver = ogles2 and enable_shaders = true (the defaults on Android) sometimes crash with the following error:

ERROR[Main]: /mt/src/client/shader.cpp:121: virtual void MainShaderConstantSetter::onSetConstants(video::IMaterialRendererServices *): An engine assumption 'normal.makeInverse()' failed.

I have no minimal reproducible example for triggering the crash, but you can trigger it by playing the "Murder" minigame as seen on the "[Minigames] A.E.S." server.

The crash occurs in the code that is responsible for calculating the value of the mNormal shader uniform on GLES2. I have no idea what's going on there, maybe the math is wrong, but: The uniform is unused, and it looks like it has been unused since its introduction in #10506.

Since mNormal is unused, I simply removed it. That fixes the crash.

To do

This PR is a Ready for Review.

How to test

1. Compile Minetest with ENABLE_GLES2=TRUE and put video_driver = ogles2 and enable_shaders = true into your minetest.conf.

   OR

   Use an Android build.

2. Join the "[Minigames] A.E.S." server on the public serverlist with three clients and play the "Murder" minigame.

3. Verify that none of your clients crash during the game.

[sfan5]

lgtm