Ignore attempts to remove device usage types that were not added.

ASSERTing on this condition is not particularly helpful as other buggy
software causes BSODs. We should be able to safely ignore and succeed
such IRPs.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge pull request #6 from pauldu/ca-112939

Ignore attempts to remove device usage types that were not added.

Approved-By: Ben Chalmers <ben.chalmers@citrix.com>