I need trac to be 1 environment, so project selection page of trac is useless for my needs.
Is there any setting in trac.ini or using trac-admin to disable available projects selection page?
No, there is no such Trac option. And it is not needed to achieve, what you ask for.
Trac's organizational unit is the environment. The '-s' switch to tracd is a cheap redirect rule, to forward requests to the webserver root directory to another directory, the single Trac env. You can arrange the same with
Redirect /trac/ http://www.example.com/trac/single-env
in a .htaccess file or httpd.confRewriteRule ^trac$ /trac/single-env
You shouldn't use available client-side redirects, because they are just unreliable.