Remove panic, as proof is invalid. (#5427)

`validity_vote` function is also called from `import_statement` not only
from `import_candidate`. The proof still holds if we assume seconded
statements always come before valid statements, which should be the case
but is not actually enforced by this module at all. On top of this the
proof is not local.

Co-authored-by: eskimor <[email protected]>