Remove line from longest lines table only if needed

Checking if a line needs to be removed is faster than just trying to 
remove it.

data/core/doc/init.lua

@@ -587,7 +587,7 @@ function Doc:update_max_line_len_range(start_line, end_line)
     elseif line_len == max_length then
       line_numbers[line] = true
     else
-      line_numbers[line] = nil
+      if line_numbers[line] then line_numbers[line] = nil end
     end
   end