ddb: don't limit pindex output in 'show vmopag'

This command already prints a tremendous amount of output, and properly
obeys the pager. It no longer makes sense to arbitrarily limit the pages
that are printed, as the reader will not be aware that this has
happened.

Reviewed by: markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D37361