tests: remove sys.executable from "required tools"

In practice this doesn't appear to have been true for some time - we
reference Python using the $PYTHON variable in all the tests now
(which we have to for PyPy and Python 3), and I've been using
~/.../python.exe to test with tip of the cpython 3.6 release branch
while working on manifest tests in Python 3 and everything seems to be
just fine. The only real observable difference from this change is
that I stop getting a warning about python.exe not being a thing on
$PATH, which seems like an improvement.

tests/run-tests.py

@@ -2097,7 +2097,6 b' class TestRunner(object):'
 
     # Programs required to run tests.
     REQUIREDTOOLS = [
-        os.path.basename(_bytespath(sys.executable)),
         b'diff',
         b'grep',
         b'unzip',