Revert "Ivar += in MRI is thread safe"

This reverts commit dd1aa738.

Since this ivar is updated in a synchronize block just below,
and has to be protected from other threads
Co-authored-by: NRyuta Kamizono <kamipo@gmail.com>

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -822,12 +822,8 @@ def checkout_for_exclusive_access(checkout_timeout)
         end
 
         def with_new_connections_blocked
-          if RUBY_ENGINE == "ruby"
+          synchronize do
             @threads_blocking_new_connections += 1
-          else
-            synchronize do
-              @threads_blocking_new_connections += 1
-            end
           end
 
           yield