On the Mac, you simply drag the application to the trash. But you won't gain anything by doing that because the trial version is the full version of Lightroom. There is nothing left out. If you install from the disk then you will probably be prompted to do an update. The update will simply bring it from Lightroom 5.0 which is on the disk to Lightroom 5.7.1 which is the version that you probably downloaded as a trial.
The easiest way to get going is, when the trial screen appears, choose the option that indicates that you have a serial number. Enter the number and you will then have Lightroom licensed on your computer and ready to continue with any work that you have done.
On the other hand, if you installed Lightroom from the creative cloud then it will have to be removed because the creative cloud installation cannot be licensed with a serial number.