Hello Vyampolsky,
I've checked both the first time installation and upgrade guides that make no reference of the option you are looking for.
I've gone through a dummy install, and there is also no option to not install the help desk section, and I've also checked the switches on the setup which doesnt provide the option either.
You could disable the helpdesk menu: Configuration -> Helpdesk Settings -> General Settings
I think it would be better to try and address the actual problem, which would be the search being slower. I'm not sure that this can be attributed directly to the helpdesk feature, certainly I have not noticed a difference in search speed. The helpdesk feature will not be using much space on the database if there no data in it, and I'm not aware that it does any back end processing that could use server resource. Is it possible you have now outgrown the SQL database?
Regards, Liam O'Doherty