[notifications] Remove unread counter, use icon instead

Summary:
Text inside a small circle has a lot of problems with fitting in there
and being properly aligned.

Test Plan:
Breeze:
{F7180340, size=full}

Breeze (with D23019):
{F7178197, size=full}

Air:
{F7178312, size=full}

Oxygen:
{F7178315, size=full}

Reviewers: #vdg, #plasma, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23018