[feature-wisun] test examples: remove tls socket, it was deprecated some time ago

[artokin]

Summary of changes

Copied from #14744 .

The example was deprecated some time ago. We should not test it in CI for 5.15 branch (it was removed from master).

Impact of changes

Migration actions required

Documentation

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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)

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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

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Reviewers

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[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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

[0xc0170]

@artokin Once Travis completes, please merge to unblock other feature-wisun PRs