Kernel+LibC: Remove the isatty() syscall

This can be implemented entirely in userspace by calling tcgetattr().
To avoid screwing up the syscall indexes, this patch also adds a
mechanism for removing a syscall without shifting the index of other
syscalls.

Note that ports will still have to be rebuilt after this change,
as their LibC code will try to make the isatty() syscall on startup.

Kernel/Process.cpp

@@ -1522,16 +1522,6 @@ int Process::sys$uname(utsname* buf)
     return 0;
 }
 
-int Process::sys$isatty(int fd)
-{
-    auto* description = file_description(fd);
-    if (!description)
-        return -EBADF;
-    if (!description->is_tty())
-        return -ENOTTY;
-    return 1;
-}
-
 KResult Process::do_kill(Process& process, int signal)
 {
     // FIXME: Allow sending SIGCONT to everyone in the process group.