Remove vendoring support when building from git sources

This is difficult to support without submodule handling in bootstrap.py, because cargo will refuse
to vendor sources unless it knows the Cargo.toml files of all tools in tree. Moving vendor support
to rustbuild means that rustbuild will be built without vendoring.

Rather than trying to solve this, just remove support altogether and require
people to use `rustc-src` if they want vendoring (or run `cargo vendor` manually).

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