docs: remove duplicated sentences and a dead 4.6 reference left by the consolidation

[gautamgb]

Consolidating 4.5 and 4.6 in #33 left four places where a sentence or bullet
survives twice. Each is the same statement written two ways, so one copy can go
without losing anything.

1. The stewardship relationship is stated twice in consecutive sentences
   ("Stewardship carries the idea of maintaining an ongoing relationship with
   each dependency" then "Stewardship implies a long-term relationship with each
   dependency"). Kept the second, which carries the securability and
   first-party-code clause.

2. The stewardship question is asked twice in a row, once as "is not only" and
   once as "goes beyond". Merged into one, keeping "goes beyond" and the "today"
   from the first.

3. The instruction to assess the attributes appears twice ("To do this, assess
   SSEM principles against the dependency. Assess each SSEM principle:").
   Collapsed to one line. That line now says attribute rather than principle,
   because the five bullets under it are the SSEM attributes named in 3.2, and
   the paragraph above already says "applying SSEM attributes". Happy to drop
   that word if you would rather keep it separate from this change.

4. The first bullet under "In practice:" is pasted over itself on one line:
   - Avoid unnecessary dependencies. - Avoid unnecessary dependencies, which introduce ongoing maintenance requirements.

The consolidation also left one pointer behind, added in a second commit:

5. Section 4.1.2 sends readers to "Dependency Stewardship (Section 4.6)" for the
   ongoing trust relationship with third-party code. Since minor edits on section 4.5 #33 that number
   belongs to no section, so the reference lands past the end of chapter 4.
   Repointed at 4.5. It is the last mention of 4.6 in the document, and the
   table of contents was already correct.

No wording is added and no meaning changes. Four lines touched.