devel/libplist: Remove special casing old compilers

We no longer support the GCC 4.2 system compiler nor GCC 4.x in
general as part of the Ports infrastructure (except GCC 4.8 for
sysutils/uefi-edk2-bhyve-csm). GCC 4.4 in particular went end of
life 11 years ago.

Approved by: jhale (maintainer)