Remove `RbfCoopCloseOptionalStaging` in LND 20 or later

[guggero]

We should eventually tho remove RbfCoopCloseOptionalStaging in LND 20 or later so that we don't keep this around.

Originally posted by @ziggie1984 in #9858 (comment)

[MPins]

Hello @saubyk and @guggero, I can help with this one if you think it’s appropriate.

[saubyk]

@MPins go ahead

[MPins]

Do you think v0.21.0 release notes should include a note about the removal of RbfCoopCloseOptionalStaging in v0.22.0?