[SMS App] Don't set font size, which causes warnings to be printed

Summary:
Whenever a ConversationMessage was constructed, a warning was printed saying "Both point size and pixel size set. Using pixel size." Stop double-setting the font size and the warnings stop

This may be system-specific because I don't see where we are setting the font's point size. I will test this on a few different systems (I haven't yet) and I hope others will test it on their system!

Note that this is easiest to test when applied on top of D15979 so you can scroll up to cause more messages to be constructed

Test Plan:
Before patch: Experience many warnings when using the SMS app
After patch: Experience fewer (0?) warnings when using the SMS app

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D16769