[Xcode] Set a code sign identity

[lorentey]

I accidentally left CODE_SIGN_IDENTITY unset, which isn't right. Setting it to - is what we need for ad hoc signatures.

Also, remove some module configuration that doesn't make sense for Swift code.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [ ] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test