Update lib/bio/io/fetch.rb

Use EBI's dbfetch endpoint as default since BioRuby's endpoint has been disabled for a while now.
Updated URL of EMBL-EBI Dbfetch
removed unused variables
Incompatible change: Default server in Bio::Fetch.new is deperecated

 * Incompatible change: Default server URL in Bio::Fetch.new is deprecated.
   Users must explicitly specify the URL. Alternatively, users must change
   their code to use Bio::Fetch::EBI.

 * New class Bio::Fetch::EBI, EBI Dbfetch client. This acts the same as
   Bio::Fetch.new(Bio::Fetch::EBI::URL) with default database name
   "ena_sequence".
Incompatibile change: remove Bio::Fetch.query; add Bio::Fetch::EBI.query

 * Incompatible change: remove a class method Bio::Fetch.query because
   default server URL in Bio::Fetch is deprecated.

 * New class method: Bio::Fetch::EBI.query. This can be used as an
   alternative method of deprecated Bio::Fetch.query method.
update RDoc documents for Bio::Fetch
Issue about Bio::Fetch and BioRuby BioFetch server is resolved

 * Issue about Bio::Fetch is resolved by recent commits.

 * The BioRuby BioFetch server is deprecated. There is no hope to restart
   the service again. EBI Dbfetch server is an alternative.
documentation about incompatible changes of Bio::Fetch