Remove reserve check if splice contribution is positive

[pm47]

If remote has a positive contribution, we do not check their post-splice reserve level, because they are improving their situation, even if they stay below the requirement. Note that if local splices-in some funds in the same operation, remote post-splice reserve may actually be worse than before, but that's not their fault.

[codecov-commenter]

Codecov Report

Merging #2771 (8f5d459) into master (830335f) will increase coverage by 0.04%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2771      +/-   ##
==========================================
+ Coverage   85.79%   85.84%   +0.04%     
==========================================
  Files         216      216              
  Lines       18127    18129       +2     
  Branches      738      775      +37     
==========================================
+ Hits        15552    15562      +10     
+ Misses       2575     2567       -8     

Files	Coverage Δ
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	85.80% <100.00%> (ø)
...inq/eclair/channel/fund/InteractiveTxBuilder.scala	91.58% <100.00%> (+0.04%)	:arrow_up:

... and 5 files with indirect coverage changes