removing rlp-encoded private key string to use flow.AccountPrivateKey

[j1010001]

Testing performed:

• running unit tests locally
• running integration tests locally
• running integration load test

[codecov-commenter]

Codecov Report

Merging #1525 (8369c6b) into master (e73e19a) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1525      +/-   ##
==========================================
- Coverage   55.18%   55.13%   -0.05%     
==========================================
  Files         521      521              
  Lines       32527    32527              
==========================================
- Hits        17950    17935      -15     
- Misses      12168    12184      +16     
+ Partials     2409     2408       -1     

Flag	Coverage Δ
unittests	55.13% <ø> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
model/flow/account_encoder.go	30.95% <0.00%> (-11.91%)	⬇️
module/component/component.go	37.86% <0.00%> (-2.92%)	⬇️
admin/command_runner.go	77.19% <0.00%> (-1.76%)	⬇️
engine/collection/synchronization/engine.go	62.90% <0.00%> (ø)

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[Kay-Zee]

Nice! this brings us more inline with how more of our other tools represent the private key (i.e. the private key bytes separately from the sig/hash algs)

This feels much better to me inter-operability, with, for example, the CLI.

[janezpodhostnik]

Nice change!