Revert "Hardcode database pool size for sidekiq"

This reverts commit b6b1d12f48.

The pool size can be set with DATABASE_POOL_SIZE ENV var on heroku. I
set it to 5, so it's ok, to revert this change.

lib/travis/sidekiq.rb

@@ -7,7 +7,6 @@ require 'travis/api/workers/job_cancellation'
 require 'travis/api/workers/job_restart'
 require 'travis/support/amqp'
 
-Travis.config.database[:pool] = 5
 Travis::Database.connect
 
 if Travis.config.logs_database