Choose active implementation only once. (simdjson#1044)

Author: Vitaly Baranov

src/implementation.cpp

@@ -58,8 +58,6 @@ class detect_best_supported_implementation_on_first_use final : public implement
 
 const detect_best_supported_implementation_on_first_use detect_best_supported_implementation_on_first_use_singleton;
 
-internal::atomic_ptr<const implementation> active_implementation{&internal::detect_best_supported_implementation_on_first_use_singleton};
-
 const std::initializer_list<const implementation *> available_implementation_pointers {
 #if SIMDJSON_IMPLEMENTATION_HASWELL
   &haswell_singleton,