Remove the obsolete python3 warning from mbed-cli

[teetak01]

This warning should be removed from the next version.

[mbed] WARNING: If you're using Python 3 with Mbed OS 5.8 and earlier versions, Python errors will occur when compiling, testing and exporting

Its printed with every mbed command regardless which Mbed OS I am using.

[teetak01]

The warning is not really even correct as not even 5.11.4 works with python3. 5.11.5 should be the first version to work.

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-888

[naveenkaje]

@teetak01 Thanks for reporting this issue.

GitHub Issue Administration Note: Somehow the labels on this issue can not be edited for triage/tracking purpose. Needs to be investigated.

[bridadan]

I've also received this warning twice in the same operation. In this case, I mistakenly tried to reimport a project:

$ mbed import <project url>
C:\Users\bridan01\onedrive_arm\Documents\dev>mbed import https://github.com/ARMmbed/mbed-cloud-client-example-internal
[mbed] WARNING: If you're using Python 3 with Mbed OS 5.8 and earlier versions, Python errors will occur when compiling, testing and exporting
---
[mbed] Working path "C:\Users\bridan01\onedrive_arm\Documents\dev" (directory)
[mbed] ERROR: Directory "C:\Users\bridan01\onedrive_arm\Documents\dev\mbed-cloud-client-example-internal" is not empty. Please ensure that the destination folder is empty.
---
[mbed] WARNING: Using Python 3 with Mbed OS 5.8 and earlier can cause errors with compiling, testing and exporting

[mark-edgeworth]

#900 created