Add deprecation notice for `run_northstar.txt`

[Jan200101]

related to #508 followed up by #512

run_northstar.txt is a relic of times gone by and is no longer needed.

[itscynxx]

Actually found a bug after posting my comment, my bad

The error pops up twice

Apart from the entire twice thing, the code looks like it's good, the error is clear, and gives a proper solution

[Jan200101]

When does it occur twice? when launching with just -northstar or via NorthstarLauncher?

[itscynxx]

When does it occur twice? when launching with just -northstar or via NorthstarLauncher?

launching Titanfall 2 via Steam using -windowed -noborder, with run_northstar.txt in the Titanfall2 directory with a 1 inside it, on windows 10