CXX-377 Fix clang warnings in headers

Cherry pick of server commit db59c175c736609970b52f91e30fc2277e22b25b

Author: acmorrow

src/mongo/client/dbclientcursor.cpp

@@ -325,9 +325,6 @@ namespace mongo {
     }
 
     DBClientCursor::~DBClientCursor() {
-        if (!this)
-            return;
-
         DESTRUCTOR_GUARD (
 
         if ( cursorId && _ownCursor && ! inShutdown() ) {

src/mongo/client/dbclientcursor.h

@@ -117,7 +117,7 @@ namespace mongo {
            'dead' may be preset yet some data still queued and locally
            available from the dbclientcursor.
         */
-        bool isDead() const { return  !this || cursorId == 0; }
+        bool isDead() const { return cursorId == 0; }
 
         bool tailable() const { return (opts & QueryOption_CursorTailable) != 0; }