I assume you want to remove multiple empty lines in one step if possible. To remove a single line, there's a context menu entry.
There are two possible approaches to remove multiple lines without much hassle:
as proposed in this answer: sort the lines alphabetically so the empty lines get sorted to the start or end, then delete them.
as proposed in this blog post: use filtering to get only the blank lines displayed, then select and delete them, then remove the filter.
To remove hard line breaks inside cells automatically, you could use the search / replace functionality and regular expressions. Just search for
\n and let the replace box empty. Don't forget to activate "
Regular expressions" under More Options.... You won't see a result immediately, since Calc doesn't change the row height automatically. So you will have to set the row height using
Optimal row height... in the context menu after removing the line breaks.
It may be possible that there aren't any empty lines in the cells, so the search/replace operation may fail. This is because LibreOffice automatically applies styles from the original html tables when pasting data from the web. To make sure that it isn't just a row format issue, just select the complete sheet and select
Optimal height... from Menu