Remove logic to handle importing the right version of pytest.
'./run_tests' still works but this allows presubmit builders to test
using 'vpython3 ./run_tests'.
Google-Bug-Id: b/266734831
Change-Id: I6a543c1f4b5b4449e723095b4a70e5228b1ccd34
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/356717
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
