[dexter] Remove lit check for python 3

This is checking the version of Python used to run lit, which is not
necessarily the same as the version used to run the dexter tests.  If
the tests are run via the build/bin/llvm-lit[.py] helper script, then
that is likely to pick up whatever version of Python is on PATH.
Conventionally, this will find Python 2. CMake already checks that
Python 3 is in use and puts the path to it in the lit site config, so
this check is redundant, and Python 3 will ultimately be used to run
dexter.

Reviewers: jmorse

Differential Revision: https://reviews.llvm.org/D69724