Revert "[Traits] Package trait description and serialization"

[ahoppen]

Reverts apple/swift-package-manager#7669

This PR broke SourceKit-LSP’s tests. It’s reproducible by

1. Cloning https://github.com/apple/sourcekit-lsp.git
2. Running the testBackgroundIndexingOfSingleFile test using SOURCEKITLSP_FORCE_NON_DARWIN_LOGGER=1 swift test --filter testBackgroundIndexingOfSingleFile

Which fails with the following log output

[org.swift.sourcekit-lsp:request-0] debug 2024-06-18 12:13:37.3640 -0700
SwiftPM log: [debug]: resolving and updating 'Package.resolved'
---
[org.swift.sourcekit-lsp:request-0] debug 2024-06-18 12:13:37.3670 -0700
SwiftPM log: [debug]: evaluating manifest for 'testbackgroundindexingofsinglefile-e43cb5da' v. unknown
---
[org.swift.sourcekit-lsp:default] default 2024-06-18 12:13:37.8340 -0700
SwiftPM log: [error]: keyNotFound(CodingKeys(stringValue: "traits", intValue: nil), Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "package", intValue: nil)], debugDescription: "No value associated with key CodingKeys(stringValue: \"traits\", intValue: nil) (\"traits\").", underlyingError: nil))
---
[org.swift.sourcekit-lsp:request-0] error 2024-06-18 12:13:37.8350 -0700
Failed to generate build graph at file:///private/var/folders/9_/vrdblnzs0m762wbv2hsryrkr0000gn/T/sourcekit-lsp-test-scratch/testBackgroundIndexingOfSingleFile-E43CB5DA: The package does not contain a buildable target. 
Add at least one `.target` or `.executableTarget` to your `Package.swift`.

[ahoppen]

@swift-ci Please test

[ahoppen]

I just put this up before Franz was looking at a fix.