Binary size flag patch (#2347)

Summary:
From: D53950535

Need to remove -O2 flag for size optimization (otherwise we use both -O2 and -Os)

Pull Request resolved: #2347

Reviewed By: larryliu0820

Differential Revision: D54762602

Pulled By: lucylq

fbshipit-source-id: 62640116fd9fbff1900dcd6f08ccbebc334175cf

CMakeLists.txt

@@ -98,7 +98,7 @@ endif()
 # data into sections so they can be properly gc'd. -s: strip symbol.
 # -fno-exceptions -fno-rtti: disables exceptions and runtime type.
 set(CMAKE_CXX_FLAGS_RELEASE
-    "-O2 -ffunction-sections -fdata-sections -fno-exceptions -fno-rtti")
+    "-ffunction-sections -fdata-sections -fno-exceptions -fno-rtti")
 if(NOT APPLE)
   set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -s")
 endif()