Commit 4d514d5a60 introduced tprintf_internal
with an additional argument "level" which was removed again in commit
7dc5296fe9.
So we can now restore the original state without tprintf_internal.
Remove also the declaration of debug_window_on (it does not exist since
commit 030aae9896) and make the
configuration parameter debug_file local as it is only used by tprintf.
Signed-off-by: Stefan Weil <sw@weilnetz.de>