Remove support for legacy .swift-testing binary

[plemarquand]

Once Swift 6.0 and Xcode 15 have been finalized and released, refactor the build config generation code to assume a unified test binary.

#986 introduced support for a unified binary but retained support for running the old .swift-testing binary if the user had a 6.0 toolchain that still generated it. Once we're confident users have moved on to generating a unified binary the build config generation can be simplified and the code path for the legacy swift-testing binary can be removed.

More context in the comment here:
https://github.com/swiftlang/vscode-swift/pull/986/files#diff-ed1fc3b638d3e40c8a0266ffea55087d5c16ef1bede54e612f8c1b3f6534288bR449