(Partially) Revert BDK regtest workaround

[tnull]

In #358 we introduced a workaround for a reachable panic in BDK's chain syncing logic that would be triggered when we start syncing the node while on tip height 0, which would be the case for our previous regtest setup.

While splitting premine and distribute_funds is fine for the most part, in some test cases we are now required to premine where we previously didn't (which takes additional CI time). We should therefore at least partially revert the workaround commit as soon as BDK shipped the fix (bitcoindevkit/bdk#1601) and we upgraded to the next beta release (which we should do before our next release).

[tnull]

Dropped the workaround in #358 already.