How do I remove the text "webform content" at the top of a block?
It looks like you are including the field label in the display. If you created the block using a custom block type do the following:


Go to Admin->Structure->Block Layout->Types 
Then select your WebForm block type 
There will be a tab labeled "Manage Display"
On this page, mark the field label as hidden.


Not sure what to do if you are using the built in Webform blocks