Remove support for deprecated password-hashing algorithm

* Remove all methods implementing and checking for the old algorithm
* Document a pre-upgrade step to check for remaining obsolete passwords
* Remove config.salt

lib/tracks/config.rb

@@ -1,10 +1,6 @@
 module Tracks
   
   class Config
-    
-    def self.salt
-       SITE_CONFIG['salt']
-    end
 
     def self.auth_schemes
        SITE_CONFIG['authentication_schemes'] || []
@@ -28,4 +24,4 @@ module Tracks
     
   end
   
-end
+end