Actually remove thin from server selection

After sinatra#1624 `thin` is still left in this block, which keeps it the most preferred server in regular Ruby. Let's remove it for real this time.

lib/sinatra/base.rb

@@ -1850,7 +1850,6 @@ class << self
     server.unshift 'puma'
     server.unshift 'falcon'   if ruby_engine != 'jruby'
     server.unshift 'mongrel'  if ruby_engine.nil?
-    server.unshift 'thin'     if ruby_engine != 'jruby'
     server.unshift 'trinidad' if ruby_engine == 'jruby'
 
     set :absolute_redirects, true