t4040: remove test that succeeded for a wrong reason
"diff-tree -b --exit-code" without "--patch" exits with 0 status,
not because it finds that the two input files are equivalent while
ignoring whitespaces, but because the implied "--raw" mode always
exits with 0 when whitespace tweaking options like "-b" and "-w"
are given, which is a long-standing bug.
We are about to fix it so that "--raw" and friends report the
differences with the exit status (even though they ignore the
whitespace tweaking options when producing their output), which
will make this test, which succeeded for a wrong reason, start
failing. Remove it.
Signed-off-by: Junio C Hamano <gitster@pobox.com>