fs/9p: Remove unnecessary superblock flags

These flags just add unnecessary extra operations.
When 9p is run without cache, it inherently implements
these options so we don't need them in the superblock
(which ends up sending extraneous fsyncs, etc.).  User
can still request these options on mount, but we don't
need to set them as default.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>