Deleted dangling reference to deleted atomic_pointer.h.

Forgot one reference to atomic_pointer.h in CMakeLists.txt
from prior CL.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237870915

CMakeLists.txt

@@ -127,7 +127,6 @@ target_sources(leveldb
     "${PROJECT_SOURCE_DIR}/db/version_set.h"
     "${PROJECT_SOURCE_DIR}/db/write_batch_internal.h"
     "${PROJECT_SOURCE_DIR}/db/write_batch.cc"
-    "${PROJECT_SOURCE_DIR}/port/atomic_pointer.h"
     "${PROJECT_SOURCE_DIR}/port/port_stdcxx.h"
     "${PROJECT_SOURCE_DIR}/port/port.h"
     "${PROJECT_SOURCE_DIR}/port/thread_annotations.h"