smbd: remove comments about deprecated 'write cache size'

smbd: remove comments about deprecated 'write cache size'

The option 'write cache size' was removed since samba-4.12 version:
https://wiki.samba.org/index.php/Samba_4.12_Features_added/changed
https://git.samba.org/?p=samba.git;a=commit;h=3fea05e0
https://git.samba.org/?p=samba.git;a=commit;h=728fabea

It is supposed to remove comments about deprecated 'write cache size',
in order to avoid confusion when reading source code and documents.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15383

Signed-off-by: Jones Syue <jonessyue@qnap.com>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Fri Jun  2 09:48:17 UTC 2023 on atb-devel-224