schemachanger: remove embedded structs from yaml output in tests

[postamar]

Embedded structs in schema changer elements show up explicitly and with a separate level of nesting in data-driven test output. It's just noise, remove it.

#76776 (comment)

Jira issue: CRDB-13438

[ajwerner]

I tried and it's harder than I thought it would be. We'll suffer a layer of indentation for the next while until somebody musters the will to fight this fight.