Add -f option to rm in clear-omninames.sh

[mmurooka]

I got the following error when calling clear-omninames.sh in CI. I hope this PR will resolve this error.

+ ./clear-omninames.sh
 * Stopping omniORB name server omniNames
   ...done.
 * Starting omniORB name server omniNames
   ...done.
tail: cannot open '/var/log/omniorb-nameserver.log' for reading: No such file or directory
Error: Process completed with exit code 1.

CI log: https://github.com/mmurooka/BaselineWalkingController/runs/8114148942?check_suite_focus=true