fix(nav): remove duplicate odis-use-case-phone-number-privacy entry from the Legacy ODIS group

[GigaHierz]

The hole, and the fix

legacy/protocol/identity/odis-use-case-phone-number-privacy was listed twice in docs.json navigation: once directly under the ODIS group (line 581) and again inside its nested Use Cases sub-group (line 585), next to its sibling odis-use-case-key-hardening. The sidebar showed the page twice. This deletes the group-level copy; the nested one is the right home because its sibling lives there.

What this does NOT do / residual risk

One-line deletion in the Legacy nav block; no content, redirects or other nav touched. The whole Legacy block is removed later by #2254 — this just stops the double entry until then.

Judgement calls

Kept the nested entry, dropped the group-level one (the page is a "use case", same as its sibling). Reversible by swapping which line is deleted.

Issues

Closes #2240 — the single acceptance condition (no duplicate nav entries) is met and checked below.

Stacking / conflicts

Branched off main, independent. #2254 will delete this block wholesale; if it lands first this PR becomes a no-op and can be closed.

Verification evidence

$ python3 …  # walk navigation, count raw vs unique page entries
nav entries: 264 raw, 264 unique; duplicates: []
$ git diff --stat
 docs.json | 1 -
$ mint broken-links   # on 6ccbcb6b
success no broken links found

Before this change on main (bdf40b37): 265 raw, 264 unique.

Remaining ops steps

• none

Checklist

• Title is the commit message I want on main
• mint broken-links passes; nav has no duplicates
• No secrets in the diff