Remove untagged tuple

[arai-a]

Untagged tuple is not used.
We should remove it.

[David Teller]

The objective of this work is to:

• inline the contents of this method into its (only) caller tagged_tuple, a few lines below;
• get rid of
  • TokenReader::enter_untagged_tuple_at
  • TokenReader::exit_untagged_tuple_at
  • all the tests that test specifically these methods
• fix whatever breaks :)