lagg: Remove the member pr_num from struct lagg_proto

It is set but never used. Remove it to avoid confusion and save a
little space.

While here, use designated initializers to initialize the LAGG protocol
table. That improves readability, and it will be safer to initialize the
table if we introduce new protocols in the future.

No functional change intended.

Reviewed by: glebius
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D55124