entry: support CE_WT_REMOVE flag in checkout_entry

entry: support CE_WT_REMOVE flag in checkout_entry

'checkout_entry()' currently only supports creating new entries in the
working tree, but not deleting them.  Add the ability to remove
entries at the same time if the entry is marked with the CE_WT_REMOVE
flag.

Currently this doesn't have any effect, as the CE_WT_REMOVE flag is
only used in unpack-tree, however we will make use of this in a
subsequent step in the series.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>