How to remove the downloaded OS X El Capitan from App Store
It's actually very easy....

Using Terminal

Type the following command:

rm -rf /Applications/Install\ OS\ X\ El\ Capitan.app/

This will remove the the install and sub-folders for the El Capitan installer.

Using Finder

Open the Applications Folder by navigating to it or by pressing ShiftCommandA.  Select the Install OS X App and Control Click or right click and select Move to Trash



That's it.  The installer has been removed!