Improve sql allocated cache reuse

This change avoids unnecessary allocation of QSqlCachedResult cache.

Change-Id: Ief592ab05b50f5e328490c504af088ec74d1938f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>

src/sql/kernel/qsqlcachedresult.cpp

@@ -267,7 +267,6 @@ bool QSqlCachedResult::cacheNext()
         return false;
 
     if(isForwardOnly()) {
-        d->cache.clear();
         d->cache.resize(d->colCount);
     }