gdb: simplify remote_target::is_async_p

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.