Reuse BlockHash from validate_pow()

[dunxen]

The validate_pow() method now returns the BlockHash since rust-bitcoin
v0.26.2 thanks to jkczyz's PR (rust-bitcoin/rust-bitcoin/pull/572).

[codecov-commenter]

Codecov Report

Merging #1627 (e256b34) into main (834fe63) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1627      +/-   ##
==========================================
- Coverage   90.80%   90.79%   -0.02%     
==========================================
  Files          80       80              
  Lines       44654    44654              
  Branches    44654    44654              
==========================================
- Hits        40550    40545       -5     
- Misses       4104     4109       +5     

Impacted Files	Coverage Δ
lightning-block-sync/src/poll.rs	87.50% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	96.94% <0.00%> (-0.09%)	⬇️
lightning/src/util/events.rs	39.54% <0.00%> (+0.28%)	⬆️

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[tnull]

Nice, LGTM.