Thread: remove constructors deprecated in 5.1

[kjbracey]

Summary of changes

Remove the immediate-start constructors deprecated in Mbed OS 5.1.

Impact of changes

Code using the old constructors will no longer compile

Migration actions required

Code using immediate-start Thread constructors must be changed to use the Thread::start method.

Documentation

None

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Pull request type

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

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Test results

[] No Tests required for this change (E.g docs only update)
[X] 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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[ciarmcom]

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

[adbridge]

@bulislaw could you take a look please

[adbridge]

CI started in the meantime

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[kjbracey]

Failure seems to be a few spurious(?) serial errors on DISCO_L475VG_IOT01A. CI restarted

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts