Remove block submit_widget
minor #12439 Remove block submit_widget (ip512)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #12439).

Discussion
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Remove block submit_widget

submit_widget parent block already renders button_widget_block. btn is already added by button_widget_block. I propose to remove this block as it adds btn twice and has no other use.

Commits
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9a08b77 Remove block submit_widget