Revert "[core] Adds fog User-Agent header"

This reverts commit a07f3203a6.

The change made a dependency on Fog::VERSION which is defined in
`lib/fog` forcing more of the system to be loaded in some cases.

Github issue #1310

lib/fog/core/connection.rb

@@ -2,7 +2,6 @@ module Fog
   class Connection
 
     def initialize(url, persistent=false, params={})
-      Excon.defaults[:headers]['User-Agent'] ||= "fog/#{Fog::VERSION}"
       @excon = Excon.new(url, params)
       @persistent = persistent
     end