Fix blinded path min_final_expiry_delta check

[t-bast]

The spec says that we must return an error if:

• cltv_expiry < outgoing_cltv_value
• cltv_expiry < current_block_height + min_final_cltv_expiry_delta

For the second check, we actually tested if:

• cltv_expiry < max(outgoing_cltv_value, current_block_height) + min_final_cltv_expiry_delta

But that check should only verify that our min_final_expiry_delta requirement is fulfilled, which is unrelated to the outgoing_cltv_value.

It was redundant with the first check, which already guarantees that intermediate nodes inside the blinded path cannot cheat by using a higher cltv_expiry_delta than what the recipient intended.

For context, see lightning/bolts#1066 and https://gist.github.com/t-bast/b1371d357a2c5f3e8c09514a62db7079

[codecov-commenter]

Codecov Report

Merging #2678 (f7d58cf) into master (e7b4631) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2678      +/-   ##
==========================================
- Coverage   85.88%   85.87%   -0.01%     
==========================================
  Files         214      214              
  Lines       17591    17590       -1     
  Branches      728      716      -12     
==========================================
- Hits        15108    15106       -2     
- Misses       2483     2484       +1     

Impacted Files	Coverage Δ
...r/acinq/eclair/payment/send/PaymentInitiator.scala	86.56% <ø> (ø)
...cinq/eclair/payment/receive/MultiPartHandler.scala	93.33% <100.00%> (-0.04%)	⬇️

... and 7 files with indirect coverage changes

[thomash-acinq]

I was a bit confused at first but actually all we care about is that the delta between now and when the addHTLC expires is at least nodeParams.channelConf.minFinalExpiryDelta. So this looks good.