Concurrency: Remove superflous _SwiftConcurrencyShims imports.

The `_SwiftConcurrencyShims` module was imported `@_implementationOnly` which
was causing warnings to be emitted during the stdlib build. The module
currently serves no purpose; the only declaration it contains is a defunct
`_SwiftContext` struct which is not referenced by anything. The module needs to
continue to exist for source compatibility, though, since it is part of the
toolchain and imported publicly from other modules.

Author: tshortli

stdlib/public/Concurrency/Actor.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 /// Common marker protocol providing a shared "base" for both (local) `Actor`
 /// and (potentially remote) `DistributedActor` types.

stdlib/public/Concurrency/AsyncLet.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 // ==== Async Let -------------------------------------------------------------
 // Only has internal / builtin functions as it is not really accessible directly

stdlib/public/Concurrency/DiscardingTaskGroup.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 // ==== DiscardingTaskGroup ---------------------------------------------------
 

stdlib/public/Concurrency/Errors.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 @available(SwiftStdlib 5.1, *)
 @_silgen_name("swift_deletedAsyncMethodError")

stdlib/public/Concurrency/GlobalConcurrentExecutor.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 // None of TaskExecutor APIs are available in task-to-thread concurrency model.
 #if !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 // TODO(swift): rename the file to ExecutorJob.swift eventually, we don't use PartialTask terminology anymore
 

stdlib/public/Concurrency/SourceCompatibilityShims.swift

@@ -14,7 +14,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 @available(SwiftStdlib 5.1, *)
 extension Task where Success == Never, Failure == Never {

stdlib/public/Concurrency/Task+TaskExecutor.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 // None of TaskExecutor APIs are available in task-to-thread concurrency model.
 #if !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY

stdlib/public/Concurrency/Task.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 @_implementationOnly import SwiftConcurrencyInternalShims
 
 // ==== Task -------------------------------------------------------------------

stdlib/public/Concurrency/TaskCancellation.swift

@@ -11,7 +11,6 @@
 //===----------------------------------------------------------------------===//
 
 import Swift
-@_implementationOnly import _SwiftConcurrencyShims
 
 // ==== Task Cancellation ------------------------------------------------------