t7811: don't create unused file

The file "empty" became unused with 1c5e94f459 (tests: use
'test_must_be_empty' instead of 'test_cmp <empty> <out>', 2018-08-19);
get rid of it.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7811-grep-open.sh

@@ -113,7 +113,6 @@ test_expect_success 'modified file' '
 	subdir/grep.c
 	unrelated
 	EOF
-	>empty &&
 
 	echo "enum grep_pat_token" >unrelated &&
 	test_when_finished "git checkout HEAD unrelated" &&