papers: Retire phkmalloc paper

It has not been installed since commit cdc37953165c ("In preparation for
the removal of the roff toolchain, disconnect the") and turned up in
a search for outdated MALLOC_OPTIONS settings.

The rendered paper is available at
https://papers.freebsd.org/1998/phk-malloc

PR: 287357
Reviewed by: bapt
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50908