We're note actually using Mubex, it might be causing a crash too

A problem we are seeing is that sometimes when BAP terminates
(ie: jobs -K), we hit a deadlock while jobs are trying to cleanup,
and sometimes that might cause msfconsole to crash and terminate.
We suspect this Mubex is a contributing factor but it has been hard
to prove because it's very hard to reproduce the crash.

lib/msf/core/exploit/remote/browser_exploit_server.rb

@@ -108,7 +108,7 @@ module Msf
       if !custom_404.blank? && custom_404 !~ /^http/i
         raise Msf::OptionValidateError.new(['Custom404 (must begin with http or https)'])
       end
-      @bes_mutex = Mutex.new
+
       super
     end
 
@@ -144,14 +144,6 @@ module Msf
     end
 
 
-    # Allows a block of code to access BES resources in a thread-safe fashion
-    #
-    # @param block [Proc] Block of code to sync
-    def sync(&block)
-      @bes_mutex.synchronize(&block)
-    end
-
-
     # Returns the resource (URI) to the module to allow access to on_request_exploit
     #
     # @return [String] URI to the exploit page