- open Search and replace (CTRL+H)
- enable “Regular Expression” (circled in red)
- enter
\n\s+in the search field, clear the replace field - press “Replace all”

This will remove all newlines (\n+) followed by all whitespace (\s+) in the next line.
\n\s+ in the search field, clear the replace field
This will remove all newlines (\n+) followed by all whitespace (\s+) in the next line.