[SIC]: Remove QMail::sslCertsPath().

This appears to have been used to possibly work around a SSL bug in Qt in the
past, but now it serves no real purpose, other than cluttering the public API
with something that isn't cross-platform.

Change-Id: Iaf95560ed291fca97f981be35d9bcebcb26f41ea
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Valerio Valerio <valerio.valerio@jollamobile.com>