remove unused Swift 6 Soundness workflow (#47)

Author: rnro

.github/workflows/pull_request.yml

@@ -23,8 +23,3 @@ jobs:
     cxx-interop:
         name: Cxx interop
         uses: apple/swift-nio/.github/workflows/cxx_interop.yml@main
-
-    swift-6-language-mode:
-        name: Swift 6 Language Mode
-        uses: apple/swift-nio/.github/workflows/swift_6_language_mode.yml@main
-        if: false  # Disabled for now.