Revert cmake -DNDEBUG for non-MSVC

Summary:
Unfortunately we can't use -DNDEBUG yet since we don't properly exclude the test libraries/executables from the non-debug builds on non-MSVC platforms. Previously this was failing on Linux for every build type except `CMAKE_BUILD_TYPE=Debug`.

Reverts a48a62d
Closes https://github.com/facebook/rocksdb/pull/2595

Differential Revision: D5436182

Pulled By: ajkr

fbshipit-source-id: 062f07cc9ce06a073b66054722b27bac1890dca3

CMakeLists.txt

@@ -174,7 +174,6 @@ else()
   endif()
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
   if(NOT CMAKE_BUILD_TYPE STREQUAL "Debug")
-    add_definitions(-DNDEBUG)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -fno-omit-frame-pointer")
     include(CheckCXXCompilerFlag)
     CHECK_CXX_COMPILER_FLAG("-momit-leaf-frame-pointer" HAVE_OMIT_LEAF_FRAME_POINTER)