Detect multiple concurent writes to Dict + remove dead code (#44778)

Thanks to #38180, the removed code seems to be dead because no finalizers should be used anymore to modify dictionaries (it was dangerous). Furthermore, it may help users to detect illegal concurrent writes, since it doesn't recurse and have new error message. There should be no, or even a positive, performance effect.

base/dict.jl

@@ -223,22 +223,17 @@ end
             keys[index] = k
             vals[index] = v
             count += 1
-
-            if h.age != age0
-                # if `h` is changed by a finalizer, retry
-                return rehash!(h, newsz)
-            end
         end
     end
 
+    @assert h.age == age0 "Muliple concurent writes to Dict detected!"
+    h.age += 1
     h.slots = slots
     h.keys = keys
     h.vals = vals
     h.count = count
     h.ndel = 0
     h.maxprobe = maxprobe
-    @assert h.age == age0
-
     return h
 end