Infer return on delegate without -dip1000

[dkorpel]

Remove one of the last useDIP1000 branches hindering dip1000 by default.

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Testing this PR locally

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

dub run digger -- build "master + dmd#14142"

[thewilsonator]

Changelog?

[dkorpel]

Changelog?

It's part of the dip1000 by default transition: https://github.com/dlang/dmd/pull/14102

The larger issue here is that dip1000's story on delegates / nested functions is incomplete, both in implementation and specification. The goal is to fix that before 2.101. I'm opening this PR already to see how buildkite reacts.

[dkorpel]

@RazvanN7 This is no longer blocked by buildkite, CI is green now (except unrelated std/logger/core failures)