NCBC-625: Remove using breaking mono compilation

Motivation
----------
The System.Diagnostics namespace is not ported to Mono. This breaks the
compilation of ITimingStore class, despite the fact that the import is
not used.

Modifications
-------------
Minor: remove unused "using" clause. This leads to compilation being
possible under Mono.

Change-Id: Ief7c1bbf40fa7dc4da6d274926bfc7717a93ba49
Reviewed-on: http://review.couchbase.org/44180
Tested-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>