loader/lua: Remove pager shim

Just after 12.2 and before the stable/13 branch, kevans added lpager.c
to provide a pager interface for commands written in lua. It was merged
into 12.3. Now that 12.2 is long since EOL, we can remove the pager shim
here. Nobody needs that old loader + new lua scripts. Plus only one
command is affected.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44142

(cherry picked from commit 8b9178cd0d35ff2beafebdd51c8c44ba2b5aeb0f)