Remove usage of assume_checked()

[tvolk131]

Follow-up to #2356

Bitcoin v30 introduced the concept of checked and unchecked addresses. While migrating over from v0.29 to v0.30, I called assume_checked() wherever necessary to make the compiler happy. Now that the migration is complete, we should remove these calls and instead use require_network().

Edit: See this comment in the changelog for bitcoin v0.32.0 which indicates that the work here might not be that important.