Redundant check for frame_index==0 in the high-overhead transfer reassembler

[pavel-kirienko]

Transfers may not begin with the first frame in the presence of frame reordering:

https://github.com/OpenCyphal/pycyphal/blob/01b9a9b57143bc916bfa2fa26102a787b4e558e9/pycyphal/transport/commons/high_overhead_transport/_transfer_reassembler.py#L138-L149