Remove Python-based test runner (#748)

TC-39 agreed to remove this tool from the Test262 repository in May of
2016 [1]. It has since been re-published as a standalone project to
facilitate independent development by interested parties [2]. Remove it
from the Test262 repository.

[1] https://github.com/tc39/tc39-notes/blob/master/es7/2016-05/may-25.md
[2] https://github.com/test262-utils/test262-harness-py