Remove check in squire that prevented pasting multi-line text

[wrdhub]

fix #6525

[charlag]

Could we please mention the original reason why it was there and why it's okay to remove now in the commit message? https://github.com/fastmail/Squire/blob/949b5b227e8b820927084d54b055ced51b0cfd9f/source/Editor.ts#L260-L273