core, billing: remove unit test

The unit test for billing persistence uses timeouts/sleep to check the presence and absence of entries in the database.  This timing, however, is inherently unstable because it depends on the OS and the thread scheduler in particular.  Thus the test will sometimes fail during the build process.

However, the usefulness of this test is very limited, so I propose simply to eliminate it.

Target: master
Request: 2.8
Request: 2.7
Request: 2.6
Request: 2.2
Patch: http://rb.dcache.org/r/6556/
Require-book: no
Require-notes: no
Acked-by: Gerd
Acked-by: Pau