Remove apache_continuum_cmd_exec CmdStager flavor

It is inferred from the platform, and we don't want to override it
needlessly. :bourne is what worked during testing, but it won't always
work. Now we can override the flavor with CMDSTAGER::FLAVOR.

modules/exploits/linux/http/apache_continuum_cmd_exec.rb

@@ -58,7 +58,7 @@ class MetasploitModule < Msf::Exploit::Remote
 
   def exploit
     print_status('Injecting CmdStager payload...')
-    execute_cmdstager(flavor: :bourne)
+    execute_cmdstager
   end
 
   def execute_command(cmd, opts = {})