drm/panel: simple: Drop prepared_time

This has been superseded by RPM in commit
3235b0f2 ("drm/panel: panel-simple: Use runtime pm to avoid excessive unprepare / prepare")
and the last user of the non-RPM case has been dropped in commit
b6d5ffce ("drm/panel-simple: Non-eDP panels don't need "HPD" handling")
whatever is left in this driver is just assigned and never used.
Drop the remaining parts.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230709162253.464299-1-marex@denx.de