unpack-trees: drop unused error_type parameters

unpack-trees: drop unused error_type parameters

The verify_clean_subdirectory() helper takes an error_type parameter
from the caller, but doesn't actually use it. Instead, when it calls
add_rejected_path() it passes NOT_UPTODATE_DIR, its own custom error
type which is more specific than what the caller provides. Likewise for
verify_clean_submodule(), which always passes WOULD_LOSE_SUBMODULE.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>