IOTBTOOL-333: Fix SimpleQueue build failure on py3

[mark-edgeworth]

Description (required)

The build system was using an internal feature of the Pool class that is unavailable
in a py3 system. This would cause an exception if tool execution failed.
Offending code has now been removed and tested on py3; error is no longer produced.

Summary of change (What the change is for and why)

See above

Documentation (Details of any document updates required)

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Pull request type (required)

[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 (required)

[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 (optional)

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[0xc0170]

just making sure, even using internal function - by removing - we do not loose anything, it was not needed?

[mark-edgeworth]

just making sure, even using internal function - by removing - we do not loose anything, it was not needed?

As far as I can tell this code is not required. The documentation on the following terminate() call says that all sub processes will be removed, so there should be no need to do this explicitly. Unfortunately there is no information on why the original change was added back in 2016.
Note that this only actually happens if the toolchain has failed to build the solution (ie it is in a failed state already).

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

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

[0xc0170]

@mark-edgeworth needed anyone else from @ARMmbed/mbed-os-tools to review? otherwise this is ready