gdbusmessage: Remove arbitrary restriction on minimum D-Bus blob length

The code in `g_dbus_message_new_from_blob()` has now been fixed to
correctly error out on all truncated messages, so there’s no need for an
arbitrary programmer error if the input is too short to contain a valid
D-Bus message header.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2528
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