libdevdctl: g++9 avoid Wdeprecated-copy

g++9 now warns about having defined an assignment operator but using the
default copy constructor, or vice versa. Avoid the issue in libdevdctl
by just using the default assignment operator too.

Reviewed by: asomers, dim
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22887