t7810-*.sh: Remove redundant test

t7810-*.sh: Remove redundant test

Since commit bbc09c22 ("grep: rip out support for external grep",
12-01-2010), test number 60 ("grep -C1 hunk mark between files") is
essentially the same as test number 59.

Test 59 was intended to verify the behaviour of git-grep resulting
from multiple invocations of an external grep. As part of the test,
it creates and adds 1024 files to the index, which is now wasted
effort.

Remove test 59, since it is now redundant.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>