Editor: Remove default "layout" block attribute from WP_Block_Type::get_attributes

The merging behavior which assigns the `layout` attribute in `WP_Block_Type` gets removed.

Props aduth, TimothyBlynJacobs.
Fixes #50257.


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git-svn-id: https://core.svn.wordpress.org/trunk@47887 1a063a9b-81f0-0310-95a4-ce76da25c4cd