relnote: remove the reference to `debugstrip`

The `debug` namespace is not intended for end user and advertising it is a path
to confusion and trouble. I think we should wait for the `admin` namespace to
exists and the command to be available as `admin--strip` before we advertise it.

Differential Revision: https://phab.mercurial-scm.org/D9817

relnotes/5.7

@@ -17,9 +17,6 b''
    can be e.g. `rebase`. As part of this effort, the default format
    from `hg rebase` was reorganized a bit.
 
- * `hg strip`, from the strip extension, is now a core command, `hg
-   debugstrip`. The extension remains for compatibility.
-
  * `hg diff` and `hg extdiff` now support `--from <rev>` and `--to <rev>`
    arguments as clearer alternatives to `-r <revs>`. `-r <revs>` has been
    deprecated.