Remove `Rakefile` from `mruby-sleep` gem; fix #4702

The `Rakefile` was a leftover from the time it was an independent gem.

mrbgems/mruby-sleep/Rakefile

-MRUBY_CONFIG=File.expand_path(ENV["MRUBY_CONFIG"] || ".travis_build_config.rb")
-MRUBY_VERSION=ENV["MRUBY_VERSION"] || "1.2.0"
-
-file :mruby do
-  cmd =  "git clone --depth=1 git://github.com/mruby/mruby.git"
-  if MRUBY_VERSION != 'master'
-    cmd << " && cd mruby"
-    cmd << " && git fetch --tags && git checkout $(git rev-parse #{MRUBY_VERSION})"
-  end
-  sh cmd
-end
-
-desc "compile binary"
-task :compile => :mruby do
-  sh "cd mruby && MRUBY_CONFIG=#{MRUBY_CONFIG} rake all"
-end
-
-desc "test"
-task :test => :mruby do
-  sh "cd mruby && MRUBY_CONFIG=#{MRUBY_CONFIG} rake all test"
-end
-
-desc "cleanup"
-task :clean do
-  exit 0 unless File.directory?('mruby')
-  sh "cd mruby && rake deep_clean"
-end
-
-task :default => :compile