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

(cherry picked from commit 03d6764b384e64c0e812c1a1b9b19c0e0d45cde2)