Remove astbase and strictvisitor from dmd

[marler8997]

It looks like astbase.d and strictvisitor.d aren't being used by the dmd executable. Removing them brings the dmd binary size down from 12.5 MB to 11.5 MB.

It also looks like they weren't used before, but were inadvertently added to the compilation when we moved to build.d (see #10446 (comment)).

[dlang-bot]

Thanks for your pull request and interest in making D better, @marler8997! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10674"

[thewilsonator]

cc @RazvanN7

[RazvanN7]

Those are used for dmd as a library. So I guess that's fine.