Make all up button speak the default description

Talkback speaks "back button" for the up button in the action bar, which
causes inconsistency with other injected pages.

Remove the self-defined content description to make it speak the default
one "Navigation up button".

Fixes: 158724875
Test: rebuild, manual
Change-Id: Ief2b1410553422cd6f06bd30e00ab4c8a59a3595