Remove inclusion of core.php in timeline_inc.php

By definition an include file is part of a main script, which will
always include core.php, so it's not necessary to reference it again
in the include.

This actually caused an error on a user's system (PHP 8.2 on IIS). The
problem could not be reproduced, but an easy fix is to remove the
useless require_once.

Fixes #33906

core/timeline_inc.php

@@ -14,7 +14,6 @@
 # You should have received a copy of the GNU General Public License
 # along with MantisBT.  If not, see <http://www.gnu.org/licenses/>.
 
-require_once( 'core.php' );
 require_api( 'timeline_api.php' );
 
 define( 'MAX_EVENTS', 50 );