Remove Python implementation of PhpUnit test suite splitting

Since Quibble 1.10.0 we have been using the Composer implementation
of PhpUnit parallel test suite splitting. Since no issues have been
reported and there is no obvious further use for the Python
implementation, remove the code.

Bug: T365978
Change-Id: I5adf0bb6cbd7854628d1e80052fbec44af7069c4
4 files changed