Cellular: Fix compilation fail with APN lookup enabled

[kivaisan]

Summary of changes

Compilation failed if cellular APN lookup was enabled as stop command has been
removed.

Fixes 0d525ff.

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

[] 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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[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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

[mbed-ci]

Test run: SUCCESS

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

[0xc0170]

Tests ongoing, were restarted

cc @VeliMattiLahtela

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 3
Build artifacts