pldm: pldm-scripts: Ignore persistent objects

Changes related to b/465550214 now expose the CX8 firmware versions as
DBus objects under /xyz/openbmc_project/software/CX8_[0-3]. These
objects are incorrectly identified by pldm_update_fw.sh as zombie
bundles from a previous update, and it attempts to remove them even
though they have no Delete method and should be persistent.

This change adjusts the script to to ignore any persistent software
object that does not have a Delete method.

Tested: https://paste.googleplex.com/5339989215936512
Fusion-Link: fusion2,n/a
Platforms-Affected: platform31
Google-Bug-Id: 495924038
Change-Id: I8dc11ecc93cd282045288e7899c287d76215fde8