Make unwrapping columns slighly more efficient

[dsas]

What?

Use flatMap rather than map().flat() when unwrapping columns.

There's no change in behaviour, it's a code quality follow-up from #45666 (comment)

Why?

It's slightly more efficient

How?

As above

Testing Instructions

Exactly as #45666, to whit:

Open a Post or Page
Add a Columns block
Add a whole bunch of content to the different columns
Select the Columns block
Choose 'Unwrap' from the Columns menu
See the Columns block get replaced by the inner contents.

Screenshots or screencast

[codesandbox]

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