Don't set terminal flags twice in a row

Don't set terminal flags twice in a row

I find this odd 'set flags twice' ancient code and comment annoyingly
distracting.  It may well be that the reason for the double-set was
simply a copy/paste mistake, and that we've been doing this for
decades [1] for no good reason.  Let's just get rid of it, and if we
find a real reason, add it back with a comment explaining why it's
necessary.

[1] This double-set was already in gdb 2.4 / 1988, the oldest release
we have sources for, and imported in git.  From 'git show 7b4ac7e1ed2c
inflow.c':

   +void
   +terminal_inferior ()
   +{
   +  if (terminal_is_ours)   /*  && inferior_thisrun_terminal == 0) */
   +    {
   +      fcntl (0, F_SETFL, tflags_inferior);
   +      fcntl (0, F_SETFL, tflags_inferior);

The "is there a reason" comment was added in 1993, by:

  commit a88797b5eadf31e21804bc820429028bf708fbcd
  Author:     Fred Fish <fnf@specifix.com>
  AuthorDate: Thu Aug 5 01:33:45 1993 +0000

gdb/ChangeLog:
2017-11-06  Pedro Alves  <palves@redhat.com>

	* inflow.c (child_terminal_inferior, child_terminal_ours_1): No
	longer set flags twice in row.