cfgfilter has graduated, remove it

cfgfilter is included in the oslo.config 1.4.0.0a2 release.

No projects use the version in oslo-incubator, so we don't have to
worry about waiting for projects to adopt it.

Change-Id: I5bb5bd39c64538f6d5914c28a018e76cde0c0e8d
Merge "cfgfilter has graduated, remove it"