This error is happening for one of two reasons (possibly both):

1. You've called your class "core_calendar_renderer" which already exists in /calendar/renderer.php, and you can't have two classes with the same name.
2. Because your class is in a namespace and you've not prefixed the extended class with \, it is trying to extend itself.

class calendar_renderer extends \core_calendar_renderer {

I've changed the code and placement of file but no changes showing on frontend.  I'm trying to remove Add new event button on calendar page.

CODE:

include_once($CFG->dirroot . "/calendar/renderer.php");

class theme_boost_core_calendar_renderer extends core_calendar_renderer {

    public function add_event_button($courseid, $unused1 = null, $unused2 = null, $unused3 = null, $unused4 = null){

        return false;

    }

}