Remove zen_mode_condition from SystemUI layouts

When EnableZenModeDialog was migrated to SettingsLib, a zen_mode_condition
layout was left in SystemUI. When EnableZenModeDialog creates its
content, it inflates R.layout.zen_mode_condition, but its overlaid by
SystemUI.

As part of the color changes for dialogs, the colors and theme set in
the SystemUI version where ones that would work with QuickSettings
theme. However, as this dialog is also inflated in light mode (and QS is
always in dark mode), the internal elements didn't contrast well with
the background.

Given that this layout should have been removed 6 years ago (to only use
the one in SettingsLib), removing it now makes it that the bug doesn't
present (as all the colors are properly set in SettingsLib +
Theme.SystemUI.Dialog).

Test: manual, light and dark mode
Fixes: 297795920
Change-Id: Ie48dd21c5fec4ca9bdd44e382a88750fdc01bb2e