Fix the USB MSD greentea host side pyusb_msd.py script

[rajkan01]

Summary of changes

Removed the redundant unicode to byte string conversion from _disk_path_windows function on pyusb_msd.py script as comparing expected input disk serial number and mounted disk on host serial number of type unicode string so it can work on both python2 and python3.

Fixes #12807

Impact of changes

With these changes, USB msd greentea host-side part of the test runs under either python2 or python3.

Migration actions required

None.

Documentation

None.

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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

@evedon

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[ciarmcom]

@rajkan01, thank you for your changes.
@evedon @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 4 of 4 test jobs passed
Build number : 1
Build artifacts