Remove unrestricted powerbox chain

[expede]

While writing tests, I realised that it doesn't make sense to pass a subject: None to get_chain. This will just get you some random powerbox, so I removed the option 🤷‍♀️ It still searches the powerboxes, but you have to tell it where you want the chain to terminate.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 12.86%. Comparing base (bc0a275) to head (4913ad0).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           wrap_auto_cid_insert      #15      +/-   ##
========================================================
- Coverage                 12.90%   12.86%   -0.04%     
========================================================
  Files                        68       68              
  Lines                      5658     5660       +2     
  Branches                   2712     2714       +2     
========================================================
- Hits                        730      728       -2     
  Misses                     4319     4319              
- Partials                    609      613       +4     

Files	Coverage Δ
src/delegation/store/traits.rs	85.71% <ø> (ø)
src/invocation/agent.rs	33.33% <0.00%> (ø)
src/delegation/store/memory.rs	26.15% <0.00%> (-0.84%)	⬇️

... and 1 file with indirect coverage changes

[matheus23]

Ah okay, so if I understand this correctly, powerlines should be created outside Store APIs in the future. E.g. in the short term via Delegation::try_sign and constructing a Payload, and in the long term with a builder, I assume.