Update INSTALL-ubuntu.md
Update INSTALL-ubuntu.md

The removed paragraph was useful for a previous version of the "Updating Discourse" code above it. It seems useless now, as precise instructions are already in the code comments. In fact, it became incoherent, as we explicitily have "bluepill quit" in the code now.
tadp/learnswift,natefinch/discourse,tadp/learnswift,natefinch/discourse,tadp/learnswift