Don't blow away target/doc

The existing rustdoc behavior for working with an already-present directory is
good enough, so there's no need for cargo to help out by blowing things away.
This also makes the copy-back portion easier by just not having it.

Closes #284
auto merge of #285 : alexcrichton/cargo/fix-cargo-doc, r=wycats

The existing rustdoc behavior for working with an already-present directory is
good enough, so there's no need for cargo to help out by blowing things away.
This also makes the copy-back portion easier by just not having it.

Closes #284