rtalloc.9: remove obsolete man page

This KPI was removed in d22337254564. Note that there are a handful of
references remaining in the src tree to these rtalloc functions that
could be cleaned up by someone with more domain knowledge.

Reviewed by: pauamma (manpages), glebius, melifaro
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38188