Revert "Ivar += in MRI is thread safe"

This reverts commit dd1aa73889.

Since this ivar is updated in a synchronize block just below,
and has to be protected from other threads

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>

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

@@ -822,13 +822,9 @@ module ActiveRecord
         end
 
         def with_new_connections_blocked
-          if RUBY_ENGINE == "ruby"
-            @threads_blocking_new_connections += 1
-          else
           synchronize do
             @threads_blocking_new_connections += 1
           end
-          end
 
           yield
         ensure