Remove workaround for injecting builtins to exec scope

[martinRenou]

Fixes #339

[SylvainCorlay]

@meeseeksdev backport to 0.12.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 0.12.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 172c6c1c9af6f7534222bc195b9ce0292145603c

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #483: Remove workaround for injecting builtins to exec scope'

4. Push to a named branch :

git push YOURFORK 0.12.x:auto-backport-of-pr-483-on-0.12.x

5. Create a PR against branch 0.12.x, I would have named this PR:

"Backport PR #483 on branch 0.12.x (Remove workaround for injecting builtins to exec scope)"

And apply the correct labels and milestones.

Congratulation you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove Still Needs Manual Backport label once the PR gets merged.

If these instruction are inaccurate, feel free to suggest an improvement.