Fix custom system info value deletion

This pull request should fix the following bug: When custom system info values are removed from a system, they get deleted correctly, but the indexes are not. The advanced search thus returns the respective servers even after value deletion.

The issue can be reproduced like this: Delete a custom info value from any system like this: Systems -> System name -> Custom Info -> Edit this value -> Delete this value. This will result in value deletion but
search still returns the system even after indexed ran again (runs every 5 minutes per default).

Note that depending on what exact version of Lucene is used, Spacewalk might not be affected by the issue adressed with commit 9e59359.