:scissors: removed deprecated `Numeric#ago` and friends

Replacements:

   5.ago   => 5.seconds.ago
   5.until => 5.seconds.until
   5.since => 5.seconds.since
   5.from_now => 5.seconds.from_now

The removed tests does not affect coverage – we have equivalent test cases in
the tests for `AS::Duration`.

See #12389 for the history and rationale behind this.