Fix a dead store in configurationForSwiftPMEntryPoint(withArguments:). (#300)

This PR fixes a (harmless) dead store to
`configuration.isParallelizationEnabled` in the aforementioned function.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Running/EntryPoint.swift

@@ -136,7 +136,6 @@ func listTestsForSwiftPM(_ tests: some Sequence<Test>) -> [String] {
 /// validated the passed arguments.
 func configurationForSwiftPMEntryPoint(withArguments args: [String]) throws -> Configuration {
   var configuration = Configuration()
-  configuration.isParallelizationEnabled = false
 
   // Do not consider the executable path AKA argv[0].
   let args = args.dropFirst()