Fix to CI job definition

Summary: Sandcastle does not raise a task if assignee is specified. Removing the
assignee will force the system to use the oncall.

Test Plan: Manual

Reviewers:

CC: leveldb@

Task ID: #6968635

Blame Rev:

build_tools/rocksdb-lego-determinator

@@ -36,7 +36,6 @@ if [ ! -z $ONCALL ]; then
   {
       'type':'task',
       'triggers':[ 'fail' ],
-      'assignee':'$ONCALL',
       'priority':0,
       'subscribers':[ '$SUBSCRIBER' ],
       'tags':[ 'rocksdb', 'ci' ],