Remove AvailableBalances::balance_msat

[dunxen]

We can now (re)remove this field after the merging of #3212.

Context from original (reverted) PR authored by @wvanlint (#2476):

This PR removes the AvailableBalances::balance_msat field.

After a discussion on Discord, it was mentioned ChannelMonitor::get_claimable_balances provides a more straightforward approach to the balance of a channel, which satisfies most use cases. The computation of AvailableBalances::balance_msat is complex and originally had a different purpose that is not applicable anymore.

Its computation takes into account holding cell HTLCs and fulfills inbound-claimed HTLCs since https://github.com/lightningdevkit/rust-lightning/pull/1268/files. However, the root cause of the underflow that was the motivation of that PR does not exist anymore. At that time, the amount in send_htlc was verified against the CommitmentStats (and this commitment transaction resolves the pending HTLC in the InboundHTLCState::LocalRemoved(InboundHTLCRemovalReason::Fulfill(_)) state). Currently, we're verifying against AvailableBalances::next_outbound_htlc_limit_msat which does not resolve such pending HTLCs. I copied and reran the test in #1268 from the main branch and it is not possible to send an HTLC back anymore and cause an underflow (sending the return payment fails with Cannot send more than our next-HTLC maximum). This is likely why the test has been removed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.62%. Comparing base (eba329c) to head (59f1689).
Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3243      +/-   ##
==========================================
- Coverage   89.63%   89.62%   -0.01%     
==========================================
  Files         126      126              
  Lines      102166   102154      -12     
  Branches   102166   102154      -12     
==========================================
- Hits        91572    91555      -17     
- Misses       7869     7874       +5     
  Partials     2725     2725              

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[TheBlueMatt]

Can we mark these as #[deprecated] and then remove them in the next release?

[dunxen]

Can we mark these as #[deprecated] and then remove them in the next release?

Yeah, that'll be fair.

[dunxen]

I'll leave this one open until 125 and then open a separate deprecation PR.

[TheBlueMatt]

Can probably un-draft + rebase.