unpack-trees: remove unused error type

unpack-trees: remove unused error type

commit 08402b0409 ("merge-recursive: distinguish "removed" and
"overwritten" messages", 2010-08-11) split
    ERROR_WOULD_LOSE_UNTRACKED
into both
    ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN
    ERROR_WOULD_LOSE_UNTRACKED_REMOVED
and also split
    ERROR_WOULD_LOSE_ORPHANED
into both
    ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN
    ERROR_WOULD_LOSE_ORPHANED_REMOVED

However, despite the split only three of these four types were used.
ERROR_WOULD_LOSE_ORPHANED_REMOVED was not put into use when it was
introduced and nothing else has used it in the intervening decade
either.  Remove it.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>