Concurrency: Remove superfluous $BuiltinExecutor guards.

Author: tshortli

stdlib/public/Concurrency/Executor.swift

@@ -319,7 +319,6 @@ extension SerialExecutor {
 @available(SwiftStdlib 5.1, *)
 @frozen
 public struct UnownedSerialExecutor: Sendable {
-  #if compiler(>=5.5) && $BuiltinExecutor
   @usableFromInline
   internal var executor: Builtin.Executor
 
@@ -329,22 +328,16 @@ public struct UnownedSerialExecutor: Sendable {
   public var _executor: Builtin.Executor {
     self.executor
   }
-  #endif
 
   @inlinable
   public init(_ executor: Builtin.Executor) {
-    #if compiler(>=5.5) && $BuiltinExecutor
     self.executor = executor
     #endif
   }
 
   @inlinable
   public init<E: SerialExecutor>(ordinary executor: __shared E) {
-    #if compiler(>=5.5) && $BuiltinBuildExecutor
     self.executor = Builtin.buildOrdinarySerialExecutorRef(executor)
-    #else
-    fatalError("Swift compiler is incompatible with this SDK version")
-    #endif
   }
 
   /// Opts the executor into complex "same exclusive execution context" equality checks.
@@ -376,7 +369,6 @@ public struct UnownedSerialExecutor: Sendable {
 @available(SwiftStdlib 6.0, *)
 @frozen
 public struct UnownedTaskExecutor: Sendable {
-  #if $BuiltinExecutor
   @usableFromInline
   internal var executor: Builtin.Executor
 
@@ -386,13 +378,10 @@ public struct UnownedTaskExecutor: Sendable {
   public var _executor: Builtin.Executor {
     self.executor
   }
-  #endif
 
   @inlinable
   public init(_ executor: Builtin.Executor) {
-    #if $BuiltinExecutor
     self.executor = executor
-    #endif
   }
 
   @inlinable