async_hooks: don't set hook_fields[kTotals] to 0

This commit removes the setting of hook_field[kTotals] to szero in
AsyncHook's enable function.

As far as I can tell this would not be required if the setting of
this field is done with the assignment operator instead of using the
addition assignment operator.

lib/async_hooks.js

@@ -74,12 +74,11 @@ class AsyncHook {
       return this;
 
     const prev_kTotals = hook_fields[kTotals];
-    hook_fields[kTotals] = 0;
 
     // createHook() has already enforced that the callbacks are all functions,
     // so here simply increment the count of whether each callbacks exists or
     // not.
-    hook_fields[kTotals] += hook_fields[kInit] += +!!this[init_symbol];
+    hook_fields[kTotals] = hook_fields[kInit] += +!!this[init_symbol];
     hook_fields[kTotals] += hook_fields[kBefore] += +!!this[before_symbol];
     hook_fields[kTotals] += hook_fields[kAfter] += +!!this[after_symbol];
     hook_fields[kTotals] += hook_fields[kDestroy] += +!!this[destroy_symbol];