Remove auto-cloning of Phobos on start

[wilzbach]

Auto-cloning is useful, but not if you run make -f posix.mak clean.

This should now be possible due to #2069

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[wilzbach]

CC @CyberShadow - I think you are going to like this ;-)

[CyberShadow]

IIRC this (or a similar change?) was added there because of some difficulties with calculating dependencies. With that in mind do you think this could break anything?

[wilzbach]

IIRC this (or a similar change?) was added there because of some difficulties with calculating dependencies. With that in mind do you think this could break anything?

Yeah you are right. I even added it myself after someone complained about the build process failing due to non-existent repos: #1827

Anyhow - I just tested this with a build from scratch and it looks like everything is working fine and we only need to depend on DMD with the shell trick as include osmodel.mak doesn't work otherwise.
PHOBOS_LATEST_FILES is a relict from the times where we copied Phobos entirely for the writeln pipeline.

[CyberShadow]

Cautiously optimistic that this won't break things in ways that are not currently obvious...