irc/inspircd: Modernize port (adding files/pkg-deinstall.in)

Addition to r456675: the new file files/pkg-deinstall.in has
been approved, but hasn't been added in r456675.

PR: 224041
Submitted by: Dmitri Goutnik <dg@syrec.org> (maintainer)
Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D13345