libevent2021: remove stale owner.txt file

I'm guessing this directory was accidentally left in the tree when
creating the owner.txt files.

opal/mca/event/libevent2021/owner.txt

@@ -1,7 +0,0 @@
-#
-# owner/status file
-# owner: institution that is responsible for this package
-# status: e.g. active, maintenance, unmaintained
-#
-owner:INTEL
-status: obsolete?