Remove impl-based MemSlice constructor.

The last uses of it have been removed from the codebase.

PiperOrigin-RevId: 702751288

quiche/common/platform/api/quiche_mem_slice.h

diff --git a/quiche/common/platform/api/quiche_mem_slice.h b/quiche/common/platform/api/quiche_mem_slice.h
index 23accb0..fa6382d 100644
--- a/quiche/common/platform/api/quiche_mem_slice.h
+++ b/quiche/common/platform/api/quiche_mem_slice.h
@@ -39,14 +39,6 @@
                  quiche::SingleUseCallback<void(const char*)> done_callback)
       : impl_(buffer, length, std::move(done_callback)) {}
 
-  // Ensures the use of the in-place constructor (below) is intentional.
-  struct InPlace {};
-
-  // Constructs a QuicheMemSlice by constructing |impl_| in-place.
-  template <typename... Args>
-  explicit QuicheMemSlice(InPlace, Args&&... args)
-      : impl_{std::forward<Args>(args)...} {}
-
   QuicheMemSlice(const QuicheMemSlice& other) = delete;
   QuicheMemSlice& operator=(const QuicheMemSlice& other) = delete;