Remove unnecessary pack_fd global in fast-import.

Remove unnecessary pack_fd global in fast-import.

Much like the pack_sha1 the pack_fd is an unnecessary global
variable, we already have the fd stored in our struct packed_git
*pack_data so that the core library functions in sha1_file.c are
able to lookup and decompress object data that we have previously
written.  Keeping an extra copy of this value in our own variable
is just a hold-over from earlier versions of fast-import and is
now completely unnecessary.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>