Update value_util.h

Author: schmidt-sebastian

Firestore/core/src/model/value_util.h

@@ -37,9 +37,6 @@ namespace model {
 class DocumentKey;
 class DatabaseId;
 
-// A bit pattern for our canonical NaN value. Exposed here for testing.
-ABSL_CONST_INIT extern const uint64_t kCanonicalNanBits;
-
 /**
  * The order of types in Firestore. This order is based on the backend's
  * ordering, but modified to support server timestamps.