git-reset --hard and git-read-tree --reset: fix read_cache_unmerged()

When invalidating unmerged entries in the index, we used to set
their ce_mode to 0 to note the fact that they do not matter
anymore which also made sure that later unpack_trees() call
would not reuse them.  Instead just remove them from the index.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>