CPPDriver/dbclient.cpp remove unused variable

This patch removes the unused variable from the exists() function in dbclient.cpp

Signed-off-by: Matt Kangas <matt.kangas@10gen.com>
SERVER-6400 FieldRef has a equalsDottedField to compare it to a full string
SERVER-6400 Use an abstraction layer into MatchExpression for different formats
   currently BSONObj vs index keys
SERVER-6400: small chnages for jstests/(and|or)*.js
SERVER-6400: cleaning api for performance
SERVER-6400: more caching of FieldRefs
SERVER-6400: FieldRef support for sub dottedField
SERVER-6400: make Matcher micro benchmark at all useful by removing ASSERT
SERVER-6400: optimizations around parsing paths
SERVER-6400: remove MatchCategory concept