Drop d_dump from struct cdevsw

It appears to be unused. These days struct disk has a d_dump member,
which is what gets passed to the kernel dump framework.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35172

(cherry picked from commit 0f50da2e09b7dddad25953d0c167efe09f7282eb)