Cellular: Remove deprecated CellularDevice::stop()

[kivaisan]

Summary of changes

Cellular: Remove deprecated CellularDevice::stop()

Impact of changes

Removed method CellularDevice::stop()

Migration actions required

CellularDevice::shutdown() should be used instead.

Documentation

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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

@ARMmbed/mbed-os-wan

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[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
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