[Mojo] Remove MessageView::TakeMessage() in channel_posix

The function is unused.

Bug: None
Change-Id: Iacc20257e5636af4b38644ec7ab8c099d0310975
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275925
Commit-Queue: Anand K Mistry <amistry@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Anand K Mistry <amistry@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#784320}

mojo/core/channel_posix.cc

@@ -70,7 +70,6 @@ class MessageView {
   std::vector<PlatformHandleInTransit> TakeHandles() {
     return std::move(handles_);
   }
-  Channel::MessagePtr TakeMessage() { return std::move(message_); }
 
   void SetHandles(std::vector<PlatformHandleInTransit> handles) {
     handles_ = std::move(handles);