cut remaining occurrences of names ReadHolder, WriteHolder

Summary: With one exception, they are all unused now that all uses have been replaced by direct use of `std::unique_lock` for `WriteHolder` and `std::shared_lock` for `ReadHolder`.

Differential Revision: D53491909

fbshipit-source-id: 7f8f4ba878799c9142c442b62950d0dd605683f8

cachelib/common/Mutex.h

@@ -224,8 +224,6 @@ class PThreadSpinLock {
 // to template between using a RW mutex and a mutex
 struct RWMockLock {
   using Lock = folly::MicroSpinLock;
-  using ReadHolder = std::shared_lock<RWMockLock>;
-  using WriteHolder = std::unique_lock<RWMockLock>;
 
   void lock() { l_.lock(); }
   bool try_lock() { return l_.try_lock(); }