read-cache: drop unused parameter from threaded load

read-cache: drop unused parameter from threaded load

The load_cache_entries_threaded() function takes a src_offset parameter
that it doesn't use. This has been there since its inception in
77ff1127a4 (read-cache: load cache entries on worker threads,
2018-10-10).

Digging on the mailing list, that parameter was part of an earlier
iteration of the series[1], but became unnecessary when the code
switched to using the IEOT extension.

[1] https://public-inbox.org/git/20180906210227.54368-5-benpeart@microsoft.com/

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>