[swift-settings] Now that we aren't using it immediately, remove it from tree.

We can always get it back from the git history.

rdar://150695113
(cherry picked from commit 9d59dbed17)

Conflicts:
	include/swift/AST/DiagnosticsSema.def
	include/swift/Basic/Features.def
	test/abi/macOS/arm64/concurrency.swift
	test/abi/macOS/x86_64/concurrency.swift

lib/Serialization/Serialization.cpp

@@ -5193,9 +5193,6 @@ public:
         case BuiltinMacroKind::IsolationMacro:
           builtinID = 2;
           break;
-        case BuiltinMacroKind::SwiftSettingsMacro:
-          builtinID = 3;
-          break;
         }
         break;
       }