Don't add extra_cfg twice.

extra_cfg is now passed to the cargo build and should become part of
the crate cfg, so it doesn't need to be added again afterwards. Use it
in the same way for metadata-only mode.

Test: Ran on openssl and memoffset crates
Change-Id: I402d6c8825de12a7faaf65e645903423e006f1e0
2 files changed