1 create a text file and call it:
gamemodes.txt
2 edit it with notepad or any other text program and write this code:
r_flashlightmuzzleflash 0
3 save changes
4 place that file that you created named gamemodes.txt, in the following path:
C: \ Program Files (x86) \ Steam \ steamapps \ common \ Left 4 Dead 2 \ left4dead2 \ scripts
that's all.