The Codex installer automatically detects if your system has Adobe AIR installed. If not, the Codex installer will automatically install Adobe AIR as well.
When you uninstall Codex, Adobe AIR is not automatically uninstalled. This is because you may have other applications that make use of Adobe AIR.
If you are sure that Codex is the only application making use of Adobe AIR, then after uninstalling Codex, you may also want to uninstall Adobe AIR.
To uninstall Adobe AIR:
On Windows, open the Control Panel and select Uninstall or change a program, then select Adobe AIR.
On Mac, run the uninstaller located in /Applications/Utilities/Adobe Installers.