Reuse rpcv6 BlockWithTxHashes for v7

[hudem1]

First, please review PRs: #2447 and #2448, as this PR depends on them.

Then,

1. v7:

• reuses the same type BlockWithTxHashes struct as v6, so removed it.
• not exactly same logic even if similar, so can't reuse v6's logic

2. v8: nothing for it as BlockWithTxHashes struct uses Header struct, which has an additional field. So, can't remove v8's logic to reuse v6's.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.79%. Comparing base (0ddfa64) to head (8a13e1b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2449      +/-   ##
==========================================
+ Coverage   73.78%   73.79%   +0.01%     
==========================================
  Files         138      138              
  Lines       16925    16925              
==========================================
+ Hits        12488    12490       +2     
+ Misses       3571     3568       -3     
- Partials      866      867       +1     

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