Mk/bsd.port.mk: remove developer recommendations to apply pathfix

The framework is already dealing with making sure the .pc files in
lib/pkgconfig are moved to libdata/pkgconfig since 2017
The other location where .pc files can be installed (share/pkgconfig)
is valid since 2022, so pathfix to deal with .pc files is now useless.

Reviewed by: mat (over matrix)
Differential Revision: https://reviews.freebsd.org/D51476