Remove `style` attribute on html nodes when pasting

[maxme]

Copy/paste with formatting works ok but we should think about removing the style and class attributes that come from the copied source. This will likely not work on the targeted site and this is a source of issues that makes paste with formatting a broken experience.

Demo video: https://bia.is/s/sTUo/paste-with-formatting-aztec.webm

[maxme]

This related to this comment: https://github.com/wordpress-mobile/AztecEditor-Android/issues/829#issuecomment-517957383 (style="whitespace: pre").