Remove unneeded volatile qualifier in fmgr.c.

Remove unneeded volatile qualifier in fmgr.c.

Currently, the save_nestlevel variable in fmgr_security_definer()
is marked volatile.  While this may have been necessary when it was
used in a PG_CATCH section (as explained in the comment for PG_TRY
in elog.h), it appears to have been unnecessary since commit
82a47982f3, which removed its use in a PG_CATCH section.

Author: Julien Rouhaud <[email protected]>
Reviewed-by: Bertrand Drouvot <[email protected]>
Discussion: https://postgr.es/m/Z6xbAgXKY2L-3d5Q%40jrouhaud