If you edit files like gamemode_casual or gamemode_competitive, they will be overwritten each time CS:GO updates, that's why Hamilton5 suggested you copy all the values in whichever you are using (gamemode_casual in your case) into a new file called gamemode_casual_server.cfg in the same directory. Once you do that, the game will automatically read from that file.
Bots and the removal of them are still somewhat of a problem in CS:GO. I have to use either the botquotafix plugin or our own custom variation of it in all of my community's servers. Below is a link to that plugin. It's a very small and simple plugin, and it works. Make sure you add the cvars in the thread's original post to your server.cfg or autoexec.cfg with the values you wish them to have.
CS:GO botquotafix plugin
https://forums.alliedmods.net/showthread.php?t=195000
EDIT: In the OP of this plugin link above, the author mentions it being specific for use with deathmatch. Though it may have been intended that way, it is a universally applicable plugin. I have it running (or variations of it's essential functionality) on idle servers, standard dust2+vote competitive pubs mingling with other plugins, our 5v5 competitive/practice server which has an variety of plugins and functions, survival games server, kz server, and I've had it running on deathmatch as well in the past. So it will work for just about any type of server you're running.