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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines