Remove nanostack-border-router example from list of supported examples

[adbridge]

Summary of changes

Nanostack border router example is now owned by Pelion and has been moved out of the ARMmbed organisation. Thus this example needs to be removed from the supported examples on the 5.15 branch.

Impact of changes

This example will not be automatically updated when we make a 5.15 release and will not be tested in the Mbed OS CI.

Migration actions required

None

Documentation

---

Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

---

Reviewers

---

[ciarmcom]

@adbridge, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 10 of 10 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 10 of 10 test jobs passed
Build number : 2
Build artifacts