ext4: documentation: remove acl and user_xattr mount options

Acl and user_xattr mount options are no longer needed since those
features are enabled by default if configured in (seee commit
ea663336). We can not easily deprecate
mount options itself (since it is probably too early), but we can
remove it from documentation first.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>