upload-pack: remove unused "create_full_pack" code in do_rev_list

upload-pack: remove unused "create_full_pack" code in do_rev_list

A bit of history in chronological order, the newest at bottom:

- 80ccaa7 (upload-pack: Move the revision walker into a separate function.)
   do_rev_list was introduced with create_full_pack argument

- 21edd3f (upload-pack: Run rev-list in an asynchronous function.)
   do_rev_list was now called by start_async, create_full_pack was
   passed by rev_list.data

- f0cea83 (Shift object enumeration out of upload-pack)
   rev_list.data was now zero permanently. Creating full pack was
   done by passing --all to pack-objects

- ae6a560 (run-command: support custom fd-set in async)
   rev_list.data = 0 was found out redudant and got rid of.

Get rid of the code as well, for less headache while reading do_rev_list.

[jc: noticed by Elijah Newren]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>