Fix unreachable-break-or-return issue in mastermind/lefdef/TimingGraph.cpp +5

Summary:
LLVM has two warnings `-Wunreachable-code-return` and `-Wunreachable-code-break` which identify `return` and `break` statements that cannot be reached. These compromise readability, can be misleading, and may identify bugs. This diff removes such statements.

`break` was often used defensively in `switch` statements. We now have `-Wimplicit-fallthrough` [enabled globally](https://fb.workplace.com/groups/fbcode/posts/8021835991186503), which replaces this inexact practice with a compiler guarantee.

The compiler used to be less intelligent and would throw an error if a function didn't return the correct type after a `throw` statement. The compiler is smarter now, so this code is unnecessary. `-Wreturn-type` is enabled globally and prevents functions from ending without an appropriate return.

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Reviewed By: dtolnay

Differential Revision: D79967119

fbshipit-source-id: 2b4bffcb80c017fbcb1b60fe1d88043f35d379cc

Author: r-barnes

third-party/mcrouter/src/mcrouter/lib/network/AsyncTlsToPlaintextSocket.h

@@ -104,7 +104,6 @@ class AsyncTlsToPlaintextSocket final : public folly::AsyncTransportWrapper {
   }
   bool isDetachable() const override {
     LOG(FATAL) << "Unimplemented";
-    return false;
   }
 
   void setSendTimeout(uint32_t sendTimeoutMs) override {