Remove invalid_from in return value of sparse chain update

[LLFourn]

78 introduced an invalid_from tuple return value to SparseChain::determine_changeset. I don't see why you need it so I removed it. I passed the test for it without it so maybe it's safe to remove :)

I'd prefer to merge this before the electrum stuff because it actually directly uses SparseChain::dertermine_changeset.

[codecov-commenter]

Codecov Report

Merging #82 (d7e600b) into master (385661d) will decrease coverage by 0.66%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   50.83%   50.16%   -0.67%     
==========================================
  Files           9        9              
  Lines         602      602              
==========================================
- Hits          306      302       -4     
- Misses        296      300       +4     

Impacted Files	Coverage Δ
bdk_core/src/chain_graph.rs	0.00% <ø> (ø)
bdk_core/src/sparse_chain.rs	78.57% <ø> (ø)
bdk_core/src/chain_data.rs	43.24% <0.00%> (-5.41%)	:arrow_down:

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