[rails] Set connection pool to 5 (#8562)

The connection pool size should be set to the number of threads per
worker. This was previously correct.

This partially reverts commit 43761fca4aa920e78b5ea6a13c690da1d2b75565.

frameworks/Ruby/rails/config/database.yml

@@ -5,7 +5,7 @@ default: &default
   password: benchmarkdbpass
   host: tfb-database
   timeout: 5000
-  pool: <%= require_relative 'auto_tune'; auto_tune.reduce(:*) %>
+  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
 
 development:
   <<: *default