What is this install icon on the launcher on my installed system?
You should not have the Install icon anywhere on the installed system, regardless of whether it was a beta, an RC or the final release. The installer and associated packages are usually removed at the end of installation.
Try removing ubiquity with
sudo apt-get purge ubiquity*
Ubiquity is the Ubuntu installer. Instead of being removed, for reasons unknown, it must have been retained after the installation. Removing its packages manually seems to be a logical step.