Remove Qt version checks.

Qt 5.3 is the minimum requirement these days. Remove all fallback code
from sources and project files.

Change-Id: If6188a471197acadda4d6baee71804ba1a8026c6
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
(cherry picked from qtcreator/847f10e9ccc8c3541782a790e04c85c6b4c701da)
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

qmake/library/proitems.cpp

@@ -212,11 +212,7 @@ ProString &ProString::prepend(const ProString &other)
 ProString &ProString::append(const QLatin1String other)
 {
     const char *latin1 = other.latin1();
-#if QT_VERSION >= QT_VERSION_CHECK(5, 0, 0)
     int size = other.size();
-#else
-    int size = strlen(latin1);
-#endif
     if (size) {
         QChar *ptr = prepareExtend(size, 0, m_length);
         for (int i = 0; i < size; i++)