tests/run-internalbench.py: Remove old CPython reference.

At one point it was possible to internal_bench CPython vs MicroPython, but
seemingly not any more.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tests/run-internalbench.py

@@ -8,16 +8,11 @@ import re
 from glob import glob
 from collections import defaultdict
 
-# Tests require at least CPython 3.3. If your default python3 executable
-# is of lower version, you can point MICROPY_CPYTHON3 environment var
-# to the correct executable.
 if os.name == "nt":
-    CPYTHON3 = os.getenv("MICROPY_CPYTHON3", "python3.exe")
     MICROPYTHON = os.getenv(
         "MICROPY_MICROPYTHON", "../ports/windows/build-standard/micropython.exe"
     )
 else:
-    CPYTHON3 = os.getenv("MICROPY_CPYTHON3", "python3")
     MICROPYTHON = os.getenv("MICROPY_MICROPYTHON", "../ports/unix/build-standard/micropython")