SQL/IBase: Don't let open() fail when lastError() is set

Since QSqlDriver::lastError() always contains the last error, it is not
a valid source to check if the last operation failed or not. Therefore
remove the check for lastError() when initializing the timezone mapping
- the function is optional since an old database might not contain the
timezone mapping table which is perfectly fine.

Fixes: QTBUG-127175
Task-number: QTBUG-125467
Change-Id: I22ea75c004654cead9a4461d2eaccfcbe8a116be
Reviewed-by: Andreas Bacher <andi.bacher@outlook.com>
Reviewed-by: Hamish Moffatt <hamish@risingsoftware.com>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 9844da5036d7a7a185ddd4eb3fa59bc4ede9e5d7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/sqldrivers/ibase/qsql_ibase.cpp

@@ -1642,11 +1642,6 @@ bool QIBaseDriver::open(const QString &db,
     setOpenError(false);
 #if (FB_API_VER >= 40)
     std::call_once(initTZMappingFlag, [d](){ d->initTZMappingCache(); });
-    if (lastError().isValid())
-    {
-        setOpen(true);
-        return false;
-    }
 #endif
     return true;
 }