Remove "unless turned on automatically" text

- In Q, the title of this preference was changed from
"Duration" to "Default duration."  The title change clarifies
that the preference controls the dnd default duration and not the
schedules, so we can now remove "unless turned on automatically"
text in the subtext.

Test: manual
Fixes: 130843902
Change-Id: I404acd61136e1e1aa4675e421c07ce871c1f9db9