Remove unnecessary race.Release annotation from Unlock.

For RWMutex we want to establish the following happens-before (HB) edges:
1. Between Unlock and the subsequent Lock.
2. Between Unlock and the subsequent RLock.
3. Between batch of RUnlock's and the subsequent Lock.

1 is provided by Release(&rw.readerSem) in Unlock and Acquire(&rw.readerSem) in Lock.
2 is provided by Release(&rw.readerSem) in Unlock and Acquire(&rw.readerSem) in RLock.
3 is provided by ReleaseMerge(&rw.writerSem) in RUnlock in Acquire(&rw.writerSem) in Lock,
since we want to establish HB between a batch of RUnlock's this uses ReleaseMerge instead of Release.

Release(&rw.writerSem) in Unlock is simply not needed.

FWIW this is also how C++ tsan handles mutexes, not a proof but at least something.
Making 2 implementations consistent also simplifies any kind of reasoning against both of them.

Since this only affects performance, a reasonable test is not possible.
Everything should just continue to work but slightly faster.

Credit for discovering this goes to Jamie Liu.

Change-Id: Ice37d29ecb7a5faed3f7781c38dd32c7469b2735
Reviewed-on: https://go-review.googlesource.com/120495
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>