tests: remove cache=unsafe from TEST-64-UDEV-STORAGE

tests: remove cache=unsafe from TEST-64-UDEV-STORAGE

mkosi switch to the newer -blockdev qemu option in systemd/mkosi#3557 [1], but
cache=unsafe is an option only -drive supports.

Since the qemu-system_x86-64 man page [2] says this, cache.writeback=on is the
default and mkosi setting the other two options to the values corresponding to
unsafe, it should be fine to drop the cache=unsafe option.

┌─────────────┬─────────────────┬──────────────┬────────────────┐
│             │ cache.writeback │ cache.direct │ cache.no-flush │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│writeback    │ on              │ off          │ off            │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│none         │ on              │ on           │ off            │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│writethrough │ off             │ off          │ off            │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│directsync   │ off             │ on           │ off            │
├─────────────┼─────────────────┼──────────────┼────────────────┤
│unsafe       │ on              │ off          │ on             │
└─────────────┴─────────────────┴──────────────┴────────────────┘

[1] https://github.com/systemd/mkosi/pull/3557
[2] https://manpages.ubuntu.com/manpages/noble/en/man1/qemu-system-x86_64.1.html