Log Message:
Use the extended PLIST_SRC support which now also reads:
PLIST.${MACHINE_ARCH:C/i[3-6]86/i386/g}
PLIST.${OPSYS}-${MACHINE_ARCH:C/i[3-6]86/i386/g}
and remove the package hack for MD PLIST files.

It was save to remove ${BLACKDOWN_ARCH} for PLIST_SRC, because it was set
to i386 in lang/blackdown-jre13/Makefile.common/Makefile.common with the
same handling we now use in bsd.pkg.mk.