kernel-install: Remove existing loader entries and UKIs
When boot counting is enabled, adding a new loader entry or UKI can conflict
with an existing one that has booted successfully and therefore has its boot
counter removed. systemd-bless-boot will fail to bless the new successful boot,
since a file without a boot counter already exists. Since kernel-install will
clobber existing files without boot counting, we should therefore remove files
without a boot count as well, when we add a file with one.
Fixes: #33504