Do not zero memory in umass_detach

The detach function is called very often, for example from
get_capacity function. We don't want to loose any pointers
here, so disable detaching for umass driver.

Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4141