gdb: simplify remote_target::is_async_p

This commit simplifies remote_target::is_async_p by removing the
target_async_permitted check.

In previous commits I have added additional assertions around the
target_async_permitted flag into target.c, as a result we should now
be confident that if target_can_async_p returns false, a target will
never have async mode enabled.  Given this, it should not be necessary
to check target_async_permitted in remote_target::is_async_p, if this
flag is false ::is_async_p should return false anyway.  There is an
assert to this effect in target_is_async_p.

There should be no user visible change after this commit.

gdb/remote.c

diff --git a/gdb/remote.c b/gdb/remote.c
index 6ecea5b..25a4d3c 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -14390,13 +14390,8 @@ remote_target::can_async_p ()
 bool
 remote_target::is_async_p ()
 {
-  struct remote_state *rs = get_remote_state ();
-
-  if (!target_async_permitted)
-    /* We only enable async when the user specifically asks for it.  */
-    return false;
-
   /* We're async whenever the serial device is.  */
+  struct remote_state *rs = get_remote_state ();
   return serial_is_async_p (rs->remote_desc);
 }