Removed unused TIMELINE_* constants

Those were added back in 2008 (6bc681f)
for an API that was never finalized, and finally removed in 2009, see
commit b3c834c.

core/constant_inc.php

@@ -594,10 +594,6 @@
 define( 'EVENT_TYPE_CHAIN', 3 );
 define( 'EVENT_TYPE_FIRST', 4 );
 
-# Timeline types
-define( 'TIMELINE_TARGETTED', 1 );
-define( 'TIMELINE_FIXED', 2 );
-
 # PHPMailer Methods
 define( 'PHPMAILER_METHOD_MAIL', 0 );
 define( 'PHPMAILER_METHOD_SENDMAIL', 1 );