Remove File.each_line method that returns an iterator

[asterite]

Fixes #6087

There's already IO#each_line which returns an iterator, so one can simply do:

File.open(filename) do |file|
  file.each_line.more_iterator_methods
end

and be sure the file is closed.

And I was wrong, there's no class method File.each_line in Ruby.

[RX14]

I was waiting to merge this until after 0.25.2 since it's a breaking change.