Remove (one last?) __sh__.

Zombies everywhere! I think this is the last one that isn't in third-party
code, though I'm pretty sure I've said that before.

Change-Id: I07c88a07c694eea646bee85c5e9ad3004b3a03e1
diff --git a/vm/Atomic.cpp b/vm/Atomic.cpp
index bd2255b..ccbf64a 100644
--- a/vm/Atomic.cpp
+++ b/vm/Atomic.cpp
@@ -42,9 +42,6 @@
 #define NEED_PTHREADS_QUASI_ATOMICS 1
 #endif
 
-#elif defined(__sh__)
-#define NEED_PTHREADS_QUASI_ATOMICS 1
-
 #else
 #error "Unsupported atomic operations for this platform"
 #endif