Remove duplicate of Unqual implementation

[edi33416]

druntime's core.internal.traits.Unqual is a copy of the same code. This PR removes the code duplication and the required double maintenance of Unqual.

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Testing this PR locally

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[andralex]

See how this works with ddoc:

public import core.internal.traits : Unqual;

[wilzbach]

We don't we do this properly and move the entire traits package to druntime? Would be a good opportunity to clean it up too...

[jmdavis]

We don't we do this properly and move the entire traits package to druntime? Would be a good opportunity to clean it up too...

And would be the gain in that? In general, druntime should only have what it needs to have. Phobos is the standard library, not druntime. If something needs to be in druntime, then we can put it there, and then we can look at doing something like we've done here, but druntime is supposed to be small, and we've generally only put stuff there because it had to be there to support the runtime. Moving all of the std.traits there doesn't help with that at all, and druntime doesn't need everything from std.traits.