fuse: remove unneeded offset assignment when filling write pages

With the change in aee03ea7ff98 ("fuse: support large folios for
writethrough writes"), this old line for setting ap->descs[0].offset is
now obsolete and unneeded. This should have been removed as part of
aee03ea7ff98.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Fixes: aee03ea7ff98 ("fuse: support large folios for writethrough writes")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>