Remove assignment to variable which is never read

Highlighted by static code analysis.

Change-Id: I0e0d33c66c3aac5135204212bea2d5d48bad3739
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 0f4b2ccea4a8701a83c37c612b0ad0e60063b7c0)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/kernel/qauthenticator.cpp

@@ -643,7 +643,6 @@ QHash<QByteArray, QByteArray> QAuthenticatorPrivate::parseDigestAuthenticationCh
             ++d;
         if (d >= end)
             break;
-        start = d;
         QByteArray value;
         while (d < end) {
             bool backslash = false;