smbd: Remove write cache

smbd: Remove write cache

Since this was written, our write path has changed significantly. In
particular we have gained very flexible support for async I/O, with the
linux io_uring in the pipeline. Caching stuff in main memory and then
doing a blocking pwrite nowadays does not belong into the core smbd
code. If someone wants it back, it should be doable in a VFS module.

Removes: "write cache size" parameter.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Nov 13 00:20:55 UTC 2019 on sn-devel-184