doc: remove redundant LTS/Current information in Collaborator Guide

Remove bullet points about how breaking changes are handled in Current
and LTS branches.

* This information is covered in
  https://github.com/nodejs/Release#release-plan.
* Having it here and in the above link means that the two may get out of
  sync, resulting in confusion.
* The above link appears later in the doc in the LTS section.
* Most Collaborators should not land *anything* in LTS branches and will
  almost never have a reason to land things into Current branches. They
  land stuff on the master branch. Adding this material here is
  confusing because it implies that Collaborators do land stuff on those
  branches. Save talk about those branches and how they're managed for
  later in the LTS section where it can be made clear that most
  Collaborators should not be landing code there anyway.

PR-URL: https://github.com/nodejs/node/pull/25842


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>