Remove `swift_common.toolchain_attrs`

[brentleyjones]

PiperOrigin-RevId: 559415879 (cherry picked from commit c7cf7c3c428e719b2d7ade197eb56a618cf09328)

Cherry-pick notes: Other rulesets will need to fully migrate to using toolchains, and swift_common.get_swift_toolchain in particular, with this change.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.