Revert "Now that we are only setting it once we can set it directly"

This reverts commit 461b7f3.

Author: dhh

railties/lib/rails/generators/rails/app/templates/config/puma.rb.tt

@@ -24,7 +24,8 @@
 # Any libraries that use a connection pool or another resource pool should
 # be configured to provide at least as many connections as the number of
 # threads. This includes Active Record's `pool` parameter in `database.yml`.
-threads ENV.fetch("RAILS_MAX_THREADS", 3)
+threads_count = ENV.fetch("RAILS_MAX_THREADS", 3)
+threads threads_count
 
 # Specifies the `environment` that Puma will run in.
 rails_env = ENV.fetch("RAILS_ENV", "development")