

This misses the step where you delete the windows partition - between steps 4 and 5. You have to delete the NTFS partition and resize your linux partition to fill the empty space.
You can also create a new linux partition if you wanted as a separate space to store stuff.
Many are suggesting deleting the Windows partition and resizing the Linux one, but another option is to back up your data and do a fresh Linux install.
During setup, you can delete all partitions and create new ones—ideal if you want to separate the root system and /home folder. Keeping system and user data on separate partitions makes future reinstalls easier, as your personal files can be preserved.