Remove old thunderbird-bin update script

`thunderbird-bin` appears to now use the
`maintainers/scripts/update.nix` script instead of this ruby script, so
the latter should be removed.

pkgs/applications/networking/mailreaders/thunderbird-bin/generate_sources.rb

@@ -1,46 +0,0 @@
-require "open-uri"
-
-version =
-  if ARGV.empty?
-    $stderr.puts("Usage: ruby generate_sources.rb <version> > sources.nix")
-    exit(-1)
-  else
-    ARGV[0]
-  end
-
-base_path = "http://archive.mozilla.org/pub/thunderbird/releases"
-
-Source = Struct.new(:hash, :arch, :locale, :filename)
-
-sources = open("#{base_path}/#{version}/SHA512SUMS") do |input|
-  input.readlines
-end.select do |line|
-  /\/thunderbird-.*\.tar\.bz2$/ === line && !(/source/ === line)
-end.map do |line|
-  hash, name = line.chomp.split(/ +/)
-  Source.new(hash, *(name.split("/")))
-end.sort_by do |source|
-  [source.locale, source.arch]
-end
-
-arches = ["linux-i686", "linux-x86_64"]
-
-puts(<<"EOH")
-# This file is generated from generate_sources.rb. DO NOT EDIT.
-# Execute the following command to update the file.
-#
-# ruby generate_sources.rb 45.1.1 > sources.nix
-
-{
-  version = "#{version}";
-  sources = [
-EOH
-
-sources.each do |source|
-  puts(%Q|    { locale = "#{source.locale}"; arch = "#{source.arch}"; sha512 = "#{source.hash}"; }|)
-end
-
-puts(<<'EOF')
-  ];
-}
-EOF