Your example is very specific and my not be accurate, but assuming it is ...
<space>Insert
or
<many spaces><some value>
You want to leave the single space lines alone, remove \n
from the rest.
Menu "Search" > "Replace" (or Ctrl + H)
Set "Find what" to "\n[space][space]*"
- Set "Replace with" "[nothing]"
- Enable "Regular expression"
- Click "Replace All"
Where [space] is an actual space. That takes any pattern with at line break, followed by least 2 spaces and flattens it out. Then fix the blank space on the first line.
For reference: How to use regular expressions in Notepad++