Udev rules: remove zvol compat symlinks (without the leading zvol/)

This is a potentially arguable change, because it removes some
compatibility cruft that certain systems or people may have come to rely
on (either a very long time ago, or unwisely in recent times).

On the other hand, it's been literally over a decade since OpenZFS
switched to the strategy of using opaque numbered /dev/zd* device nodes,
with the canonical zvol access path being a directory tree of symlinks
created by udev rules inside /dev/zvol/*. (See #102.) Even at the time,
the /dev/* scheme was labeled as being for "compatibility".

This commit removes the second tree of symlinks located directly at
/dev/*, under the assumption that anybody with any sense has been using
the intended /dev/zvol/* path for a very very long time now.

(The more I think about this, the more I anticipate that some large
fraction of people will have been blissfully unaware that the intention
has been for them to use the /dev/zvol/* tree all along, and they will
have come to rely upon the /dev/* tree simply because it's been there
this whole time despite being a compat thing.)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Neal Gompa <ngompa@datto.com>
Signed-off-by: Justin Gottula <justin@jgottula.com>
Closes #12303