I was having the same problem with OTA apps I was making. Before I solved the problem, the only way to remove the "Installing..." icon was to do a factory reset on the device (unless its jailbroken).
I changed the bundle identifier string in the .plist file for my OTA app to match the "id" of the app.xml. This made it so there was no longer two separate icons while it downloaded the app and nothing leftover on my dashboard after the install.