dir: revert work-around for retired dangerous behavior

dir: revert work-around for retired dangerous behavior

directory_exists_in_index_icase() dangerously assumed that it could
access one character beyond the end of its directory argument, and that
that character would unconditionally be '/'.  2eac2a4c (ls-files -k: a
directory only can be killed if the index has a non-directory,
2013-08-15) added a caller which did not respect this undocumented
assumption, and 680be044 (dir.c::test_one_path(): work around
directory_exists_in_index_icase() breakage, 2013-08-23) added a
work-around which temporarily appends a '/' before invoking
directory_exists_in_index_icase().

Since the dangerous behavior of directory_exists_in_index_icase() has
been eliminated, the work-around is now redundant, so retire it (but not
the tests added by the same commit).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>