Revert "Create new target for common code (#1331)"

[kateinoigakukun]

This reverts #1331 to repair CMake build on Windows CI

https://ci-external.swift.org/job/swift-PR-windows/47966/console

[kateinoigakukun]

@swift-ci test

[d-ronnqvist]

It was my understanding that issues with the Windows CMake build would not be blocking anything. If that's not correct, can we revert #818 instead?

[kateinoigakukun]

@d-ronnqvist Unfortunately it actually blocks merging PRs on swiftlang/swift because we use the CMake build setup for Windows CI and it's a mandatory check.
https://ci-external.swift.org/job/swift-PR-windows/

Given that it's affecting widely, it might be worth reverting this to get the CI back to green as soon as possible 🙏 Also reverting #818 alone unlikely solve the issue because we already use the CMake setup in the CI.

[d-ronnqvist]

That's very different from how I was led to believe that the DocC Windows CMake configuration would work at this early stage.

Because we have no CI for that ourselves and insufficient instructions on how to locally update and locally verify changes, I would suggest that we revert #818 and any CI configuration that relies on it.