Remove an extra `@mutex.synchronize`

Since `@mutex.synchronize` is enforced in the `ensure_connection!` method,
there is no need to do so on the caller side.
https://github.com/jondot/sneakers/blob/c1b47f9c5d5a95da728bbe1700795790e4efbb12/lib/sneakers/publisher.rb#L22-L26

Due to this, `ThreadError(deadlock; recursive locking)` has occurred.

activejob/test/support/integration/adapters/sneakers.rb

 # frozen_string_literal: true
 
 require "sneakers/runner"
-require "sneakers/publisher"
 require "timeout"
 
-module Sneakers
-  class Publisher
-    def safe_ensure_connected
-      @mutex.synchronize do
-        ensure_connection! unless connected?
-      end
-    end
-  end
-end
-
 module SneakersJobsManager
   def setup
     ActiveJob::Base.queue_adapter = :sneakers
@@ -80,7 +69,7 @@ def can_run?
     def bunny_publisher
       @bunny_publisher ||= begin
         p = ActiveJob::QueueAdapters::SneakersAdapter::JobWrapper.send(:publisher)
-        p.safe_ensure_connected
+        p.ensure_connection!
         p
       end
     end