t/t7704-repack-cruft.sh: consolidate `write_blob()`

t/t7704-repack-cruft.sh: consolidate `write_blob()`

A previous commit moved a handful of tests from a different script into
t7704, including one that relies on generating random blobs.

Incidentally, the original home of this test defined its own helper
"write_blob" for doing so, which is identical in function to our
"generate_random_blob" (and is slightly inferior to the latter, which
cleans up after itself).

Rewrite the test that uses "write_blob" to no longer do so and then
remove the function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>