sysutils/heirloom: Fix build on 15-CURRENT after e24a65528388

src/e24a65528388 was one of a series of commits to 15-CURRENT that
removed kernel stack swapping support. e24a65528388 removed the
P_SWAPPINGOUT flag resulting in this port failing to build on
15-CURRENT. Remove the test for the flag.

MFH: 2024Q3