[mojo][doc] Remove reference to GetRemoteInterfaces

This change removes the reference to RenderFrame::GetRemoteInterfaces()
since it's no longer used in chromium.

Change-Id: Idd28e276b25ff08cf71c3656d0a52a413a62f5dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067857
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743649}

docs/mojo_ipc_conversion.md

@@ -216,8 +216,7 @@ decide how to proceed:
           `RenderFrameHostImpl` and `RenderFrameImpl`.
         - If the message is sent from a renderer to the browser:
             - If an existing interface is bound by `RenderFrameHostImpl` and
-              acquired either via `RenderFrame::GetRemoteInterfaces` or
-              `RenderFrame::GetBrowserInterfaceBroker` and the interface seems
+              acquired via `RenderFrame::GetBrowserInterfaceBroker` and the interface seems
               to be a good fit for this message, add the equivalent Mojo message
               to that interface.
             - If no such interface exists, consider adding one and registering it