[phb]: remove references to compiler:openmp

Uses=openmp was removed in 07fb2d5e9d001934f1670d01aec1f536f14ebde2 in ports[1].

Remove references to this option in the Porter's Handbook.

[1] https://cgit.freebsd.org/ports/commit/?id=07fb2d5e9d001934f1670d01aec1f536f14ebde2