Remove the problematic and unneeded AST_MODFLAG_GLOBAL_SYMBOLS from pbx_lua.c

Remove the problematic and unneeded AST_MODFLAG_GLOBAL_SYMBOLS from pbx_lua.c

AST_MODFLAG_GLOBAL_SYMBOLS was causing the module to be incorrectly loaded
before pbx_config.  pbx_config was therefore blowing away contexts that were
created by pbx_lua.  With AST_MODFLAG_DEFAULT the load order is now correct
and contexs are being properly merged.  AST_MODFLAG_GLOBAL_SYMBOLS was not
needed anyway since no other modules needed its global symbols that early.

ASTERISK-23818 #close
Reported by: Dennis Guse
Tested by: Dennis Guse
Tested by: George Joseph

Review: https://reviewboard.asterisk.org/r/3629/
........

Merged revisions 416668 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@416669 65c4cc65-6c06-0410-ace0-fbb531ad65f3