shared/usb: reply unsupported to PREVENT_ALLOW on all LUNs

Per @tannewt review on #10967: tinyusb advertises is_removable=1 for
every LUN in its default INQUIRY response (msc_device.c line 781), so
the per-LUN branch was inconsistent with what the host already sees.
Collapse to a single unconditional ILLEGAL_REQUEST reply — host keeps
polling TUR on all LUNs, so eject/re-mount works uniformly across
CIRCUITPY, SAVES, and SD.