QSqlDriver functions made virtual

QSqlDriver functions made virtual

Certain QSqlDriver functions were marked to be made virtual in Qt5.
subscribeToNotification, unsubscribeFromNotification,
subscribedToNotifications, isIdentifierEscaped, and stripDelimiters.

This patch makes them virtual and removes the no longer needed
Implementation counterpart functions. It also updates the relevant
drivers. This patch has no regressions on the tests in
tests/auto/sql/kernel/, tested with sqlite and postgres.

Change-Id: Ia2e1c18dfb803531523a456eb4e710031048e594
Reviewed-by: Mark Brand <mabrand@mabrand.nl>