mm-account: Don't set SMS validity

Relative value of 168 results in the validity to be around 2 hours 50 minutes, which is very low. So instead, don't set validity time at all so that the default values are used.

Here is some details: https://www.freedesktop.org/software/ModemManager/doc/latest/libmm-glib/MMSmsProperties.html#mm-sms-properties-set-validity-relative Afaik for example KDE Spacebar does not use any validity time and they don't have this issue in Plasma. Most carriers are using something between 36-72 hours so if we need to put something maybe we should go with the longest so 4320 :)?

I changed also the branch.

~~We noticed in Pine64 forum that Chatty will reject incoming SMS if device has been offline for 2 hours and 50 minutes. This might be because Chatty sets validity period but it might not be necessary.

Another user, Kevin Kofler, did suggest that if this line will be removed then you will get SMS also after longer shutdowns.

I think this is actually quite important and other people (sender) are likely to assume that text messages will go through~~