packfile,delta: drop the `cast_size_t_to_ulong()` wrappers

packfile,delta: drop the `cast_size_t_to_ulong()` wrappers

When I started the transition from `unsigned long` to `size_t`, in the
interest of keeping the patches reviewable, I introduced these calls to
prevent data type narrowing from silently failing to handle large object
sizes. I also introduced `*_sz()` variants that would allow most of the
callers to keep using that `unsigned long` that the 90s kindly asked to
be returned.

After the preceding commits, the only places that called the narrow
wrappers either no longer exist or already use the `_sz` form
internally, so the wrappers just narrow values back through
`cast_size_t_to_ulong()` for no reason.

Drop them and rename the `_sz` variants back to the natural names.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>