Remove unheard icon next to voicemail.

Since unheard voicemails are anyway in the top "New" section, there is no
need to have the extra icon.

Moreover, this means we do not have two similar looking icons next to
each other, which make the iconography harder to understand: one meant
"unheard message", the other meant "press to play".

Bug: 5254001
Change-Id: Ie0e40a87435442a0f829c544d83ad24bd6fb7f17